At the battle of Gaugamela the Macedonians were highly outnumbered and experienced generals such as Parmenion asked Alexander to withdraw. Several years later the still unbeaten Macedonians say they will not fight despite facing much higher odds at Gaugamela. What encouraged them to fight against the Persians and what changed to refuse continuing into India?
several reasons.
the entire premise of the initial campaign was to kill darius, take over all of persia, and bring justice for the offense (Thermopylae, burning athens, etc) xerxes brought greece years earlier.
by the time alexander reached india, many of the original macedonians remaining (which were proportionately few, as alexander integrated MANY foreign mercenaries into his corps and dismissed with honors long-serving macks) felt that they had already achieved this mission of conquering persia and avenging greece. they had just suffered a horrid three year campaign in afghanistan. to what end would alexander continue? they wanted to go home and see their wives and kids one more time before they died for him -- which they all were ready and willing to.
the indian army was massive and had a large amount of elephants, which, for obvious reasons, were unsettling to troops.
the terrain in india was also trying. many died of disease and had their feet (as well as the hooves of horses) ruined by the massive monsoons. it was a foreign land in every sense of the world "foreign."
last, but also importantly, at gaugamela, macedonians loved alexander. he lead by example. he put his entrusted veteran generals (such as permenion) to hold down the left left, as they felt their sarissa center was impenetrable (which it proved to be) while alexander himself lead his companion cavalry to deal the death blow to darius.
think about it. as a soldier, to watch your king take the lead and charge into the depths of the enemy has to be one of the most invigorating feelings possible. granted, the majority (especially this left flank we speak of) could not see and had no clue what was going on on the right where alexander was stationed, but the soldiers knew. his great victories leading up to gaugamela gave this troops total faith. and, again, almost ALL of these soldiers were macedonians, hungry for revenge against persia.
by the time they were in india, over half the army was comprised of foreign mercenaries simply fighting for money.
hope this helps!
EDIT: check out this book, which is my primary source. you will love it:
http://www.amazon.com/Alexander-Great-Philip-Freeman/dp/1416592814