Question

What caused Spain and Portugal to begin the Age of Discovery?

Answer 1

The main cause of Portugal and Spain to begin the Age of Discovery  (Age of Exploration) in XV century is search for the new trade route to spice-laden East due to fall of Orthodox Christian Constantinople to the Ottoman Empire. Old trade routes being shuttered was great damage for Iberian merchants.

Answer 2

When the Byzantine empire fell to the Ottomans, the route to trading with the middle east and the a bit further east went down the drain because the Ottomans wanted to control it all. They often forbid others from trading or took large amounts of money as tax for the trade. Also, the Catholics didn't want their people going there and trading with Ottomans so that was another obstacle. This is why they decided that it was necessary to find a new route to reach India, for example, which didn't involve any dealing with Arabs or Turks. This was much more cost effective because they could trade directly with eastern people without middlemen.