Trade in general. Merchants in the west wanted new markets to sell their goods into and early exploration revealed not only that there were distant markets to sell to, those distant places had things they didn't have, had never seen before, and when they got a taste of them, they wanted some. The west had more technology (though still primitive by modern standards) and made things that couldn't me made (yet) in the east.
Trade. Making money. Everybody does, always has, always will. And when money is involved and different cultures and religions clash with one another, there can be wars. Always has been, probably always will be.
And the first trade routes ran right through those places you mentioned.