What motivated the europeans to explore a route to southeastern Asia?

Spices, especially and other Oriental goods had been known and appreciated for a long time in Europe and demand for them had always been high. Supply only came by overland route (the so-called "Silk Road") and the amounts that could be carried by camels always was insufficient. Moreover, during the travels many local rulers levied taxes on them and goods were on the way already traded and re-traded, and sometimes wars that broke out somewhere along the route stopped supply altogether for a while.

All of this lead not only to insufficient supply but also to very high prices. The Portuguese first of all quickly figured out that finding a sea-route themselves would not only cut out the many middle men, but the payload of even one ship would be much bigger than what a whole camel caravan could carry. Getting the goods themselves would at the same time allow them to corner the market through lower prices and make them still a very hefty profit. Which turned out to indeed be the case.