The chief cause of much of Europe’s exploration was trade, the one thing that binds humanity and transcends boundaries such as religion, culture and even language.
The Ancient Greeks wanted tin for their bronze helmets, the Anglo-Saxon king wanted Indian Garnets to decorate his sword, A Frankish courtier might want ivory to carve into chess pieces and so on and so on. If one wants goods, be they natural produce or manufactured, that cannot made crafted, grown or dug up locally you will have to trade to get them. Most areas of the world also have something worth exporting; Vikings carried boatloads of amber, fur and slaves off to the markets in Constantinople to return with fine silks, a decorated Buddha statue and silver to make into coins.
Trade is also one of the big factors in industrialization and urbanization. A potter might be able to sell his pottery in the local region and perhaps hire a few more people to help him keep up production, now if someone showed up on a boat ready and willing to buy a thousand pots to sell in the next town this potter might be able to expand his business and hire more and more people that live solely by manufacturing a certain item. The more people he can sell his goods too or the bigger his market is the more people can be employed in a crafting profession, something which will typically lead to greater specialization and mechanization of production.
Now I derailed myself a little bit so on to exploration;
The well known trade route to a known destination is not going to generate as much profit to traders as the new or unknown route is, it might be faster or it might reach wholly unknown populations of people to trade with. An early European example would be the Vikings. Before they went through Russia most trade to Scandinavia went through Rhine region and from there up north, Byzantine or Middle Eastern silks had to travel quite a long way to reach the north in that way. When the Vikings openend up a new trade route they could sail from the Baltic down (or up) Russia’s river network into the Black Sea and from there to Constantinople, bypassing the European middle man and making them quite rich and well connected. Later global exploration was driven by very much the same thing.
The overland exploration of people such as Marco Polo fed back information to Europe about exotic and rich lands but didn’t establish any trade routes. The silk road had been bringing spices and such into Europe since Roman times and was not really facing any competition. Goods from China and the Spice Islands were shipped to India and from there on Arab traders took them either overland to the Levant or Constantinople or across the sea right up to Egypt, from there on European merchants (chiefly Italians) imported goods into Europe and got quite rich doing so.
India registered in the mind of most Europeans while China or Cathay and the Spice Island didn’t really, this led to quite a lot of people believing India itself was the source of many of the spices that made it into Europe rather than a conduit in a continent spanning trade network. The fact that this trade route was firmly in the hands of Italians and Middle Easterners did not really sit all that well with some Western Europeans, not altogether because of some abstract concepts such as a different religion or culture but rather because it were other people making a lot of money and not them.
An alternative route to India however was not an easy objective, for a start the most prominent scholars from the Ancient world held that Africa extended downward until it connected itself again to the Eurasian continent making the Indian Ocean an enclosed sea you could not simply sail into. Secondly pretty much no one had any good idea what Africa looked like that far south. People around the Indian Ocean typically didn’t go much further south than the Horn of Africa and Europeans and North Africans didn’t venture much further south than modern day Morocco, it didn’t really help that the Atlantic ocean was much more temperamental and rough than the calm Mediterranean.