In most cases is not a matter of chance. I will try to answer the basic core of you question which remains in place even today.

Typical thing for regular armies is to move significant amount of troops from place A to B to occupy certain surface, either a valley, crossroad, province, country you name it.

There is one thing that has always and still today conditions the movement of troops and that is geography and infrastructure on that geography. In other words when you want to move armies from one point to the other you are often forced to use the existing paths, roads, railways etc.

As an example if you are to invade Russia from the West you come across the Pinsk Marshes and you have to surround them. Napoleon had to, the Russian Army invading Germany in WW1 had to and once repulsed the survivors tried to seek refuge in them, the German Army in WW2 had to get around them too separating Army Group North and Army Group South. If you want to invade Spain from the North you have to either cross the Pyrenees Mountains dividing Spain/France West-East and there are only few practicable mountain passes for an army or go to the coastal extremes in the borders, either the Atlantic or the Mediterranean. Same route leading to Baghdad by the US Army has been used for centuries for other armies and moving goods and if you are American, the Civil War theater in the East saw Lee and the Federal army marching over and over on the same roads, fording points and rivers.

If you live in Europe most modern roads run above ancient paths and roads some dating back to the Roman road system. The point is probably clear by now, you can move only along certain lines, and that applies to both parties. It means that you know the available routes and the choices are limited and simplifying you know you are going to run into each other.

At this stage is a matter of scouting and trying to conceal your movements as much as possible, but you know you will find the opposing forces either in a bridge, an oasis a crossroad etc. Therefore armies will come upon each other, and in most cases they both will know so once contact is made in all likelihood there will be a battle the next day with the first morning lights, that is why you are there after all. Once 2 armies come into contact you need to be fast to choose your ground because that is going to give you a tactical advantage.

Refusing battle is a very very difficult option, retreating leaves you exposed with little maneuvering capability and with you rear under constant pressure to be defeated in detail.

I hope this illustrates the basic premise. On to how armies give battle we should probably look into which period and opposing parties as every army has a specific organization and fighting style. Romans are different from Alexander's armies and he was from his Persian enemies. Each will try to impose his choosing of ground to make its tactics prevail. Hope it helps.

A good reading to start on this area, fascinating for me, is the book: "Battling the Elements: Weather & Terrain in the Conduct of War." by Harold Winters.