This is a great question and a fascinating one. It's always difficult to tell what the average person's life was like in antiquity. If you ever study Alexander's life in depth, you'll run into many unanswered questions and conflicting accounts. If a figure as famous as Alexander remains mired in ambiguity, imagine how tough it is to pinpoint the life of this "average man".

First, it's reasonable to conclude that the average soldier didn't know they were going to the edge of the known world. The most they knew was that, after the Greeks were suppressed, they were headed for Asia Minor. Alexander probably didn't even know he was going to end up in India.

It's also important to note that these weren't people randomly joining up. Alexander's father, Phillip II, handed him "the most perfectly organized, trained, and equipped army of ancient times", according to JFC Fuller (maybe hyperbole/exaggeration but it illustrates the point well.)

Second, death was not "certain" any more than death is "certain" in any military campaign. Why does anyone do anything? Because it's a job; because there's treasure abroad; because there's honor in battle; because your culture and society expects you to fight; because you've been raised as a warrior since birth; because they were conscripted during the campaign; because they fought as allies for political reasons or as mercenaries for monetary gain.

There's thousands of reasons why someone would fight. With the dearth of primary sources (e.g. "Dear diary, I am joining Alexander's army because XYZ") it's difficult to pinpoint what the average soldier lived like, much less what ambiguous concepts and paradigms drove them to pick up a spear and go kill people in Asia Minor.

The primary sources we do have are often Alexander's top lieutenants talking about the man himself, troop movements in the aggregate, grand strategy, and so on. When they do address the troops, it's usually as brief as "the troops were happy because we won" or "the troops were pissed for lots of reasons. Then Alexander gave a great speech that everyone loved."

Your third question is a little easier. We know a fair amount about what life was like. For instance, Alexander had this group of elite soldiers called the Heitairoi, or the "Companion Cavalry". We know that they loved to do two things: Drink and hunt. Alexander himself got some pretty serious alcohol poisoning during a big party with the Hetairoi a few weeks before he died. It's hard to tell from the sources, but they certainly seem related.

Wikipedia is a pretty good source on the Hetairoi. This article seems moderately well sourced and accurate: http://en.wikipedia.org/wiki/Companion_cavalry

We know plenty aout the more technical details; equipment, logistics, and so on. The average soldier's load, for instance, was thirty pounds. Most were on foot, but the wealthier soldiers were on horseback. Most of the gear was carried by troops, rather than servants or pack animals, which meant the army was mobile and flexible (but that also means the every individual dude was humping his own gear for thousands of miles on foot). There are lots of sources about this and they're easy to find; some of my info comes from here: http://books.google.com/books?id=OiMWqRokCZ4C&pg=PA24&lpg=PA24&dq=alexander+the+great+average+soldier&source=bl&ots=mhOgLzEmRR&sig=4wDkMY4yCakAv9UOG_RPuGDGYR8&hl=en&sa=X&ei=ECMFU9DwKYrp0AGanYCIBw&ved=0CFsQ6AEwBzgK#v=onepage&q=alexander%20the%20great%20average%20soldier&f=false

Many of these young soldiers found wives in Asia Minor, following the "conquering" of the Persian Empire. Some accounts suggest Alexander encouraged this.

Peter Sommer's writings are highly interesting. He's not really a historian, but he replicated Alexander's journey on foot. There's a documentary on YouTube where he makes a bunch of observations about what it must've been like. You can read more here: http://www.petersommer.com/travel-writing/alexander-the-great-travel-article/

Ultimately, the best place to look to is the sources. The original accounts of Alexander's lieutenants have been lost, but we have five main surviving accounts based on those lost accounts: Arrian, Curtius, and Diodorus Siculus (also Justin and Plutarch but those wouldn't help answer your question at all). These are fairly cheap. They are also probably available on google scholar, perseus or some other database.

First of all, there have been a lot of people recently on this sub using the term "Hellenic Period." There is no such thing as the Hellenic Period. Greek history in antiquity is divided into the Dark Ages, the Archaic Period, the Classical Period, and the Hellenistic Period. The Roman Period is also referred to, and the Mycenaean and Minoan Periods are often referred to, although the technically correct thing to do is to refer to them by their Helladic Bronze Age labels.

Okay, got that through with. I once had a professor who explained the purpose behind war and joining an army in the ancient world with a single sentence: booty. Obviously it's more involved than simply grabbing plunder, but for an ordinary man, even for a king the spoils of war from plunder alone are incredibly vast. The study of ancient economies has shown generally that warfare was one of, if not the singlemost, important economic activity for most ancient societies at a certain level, and even past that level of development it is still of enormous importance. The promises of sharing in the spoils of war would be incredibly important for an ordinary soldier, something that is clear again and again in accounts of armies from Hannibal to Caesar and even to Xerxes. With regards to Alexander, remember that the troops of the Phalanx probably weren't paid, unlike the mercenaries in the army, until towards the end of the campaign, when they demanded from Alexander what they were due.

But I said it was more complicated than that. Well, yes, it is. But it varies greatly from place to place. The feeling of necessity to follow a strong leader in antiquity is a pretty strong one, often descending from rituals and the traditions of ancestors, i.e. you follow your leader because it's the thing to do. Members of ancient societies don't really follow the same thought processes that we might, particularly in weighing the pros and cons of a decision like this. They'd just do it because it's considered necessary.

So I said that it varies. What about Macedon? The kingdom of Macedon was extremely backward compared to the rest of the Greek world. So backward, in fact, that it preserved social customs that had been lost in the Dark Age or even before. Yeah, that old. That archaic. In particular, scholars often stress the degree to which the Macedonian society which Philip took over resembled that of Homeric Greece. It's an analogy that can easily be taken too far, but in many ways it's very true. Macedonian society was grounded in a highly hierarchical system of barons, lords, and vassals, and while the individual man wasn't really a serf the way he might be in the Middle Ages, he still owed direct allegiance to his lord. The Macedonians of Alexander's army serving in the Phalanx would've thought of their service more or less along the same lines as the faceless masses of men standing behind the champions in Homer. They were there because their lords had gone to war and they owed their lords a duty.

That of course isn't the only reason. Human beings are complicated, and Philip's levies were taking place at a time when he was trying to restructure Macedonian society (or at least the aristocracy) in a revolutionary way. But they're certainly very important reasons and ones that would've been foremost in the minds of the soldiers. I think the most important objection that can be made to all of this is the mutiny of the soldiers. If Macedonian society was so rigid that troops would allow themselves to be conscripted (remember that these guys aren't going voluntarily, but because their lords have conscripted them) why would they revolt? For one thing, it didn't even occur to them to revolt for years, despite all the hardships they faced. Many scholars see it as an indication of just how hard these men were pushed by the end that it finally broke out in revolt, not only against their king, but against their lords as well. It's also an indication of just how rapidly Macedonian society changed during that time.

Now, I haven't really answered the question satisfactorily, and I'm aware of that. The difficulties in ascertaining a person's motives without any direct evidence are of course immense, but I what I've tried to do is steer you towards the right path in understanding the cultural and social stimuli that would've been driving these men. The best thing to do is to really acquaint yourself with the social and cultural idiosyncrasies of Macedonian society to really get a handle on the way these people thought. I recommend taking a look at Hammond--he's somewhat out of date, but his work on Macedonian society and how it influenced Alexander is quite important.