what were the political reasons for Columbus' sailing to America?

Answer 1

First of all, he wanted to find an alternative trade route to Asia in order to avoid the expensive land route which was dangerous or impossible because of the Arab presence in between.
Second, he wanted to expand the domain of the Christians in general and to spread Christendom. Since he sailed for Spain, this also translated to expanding the political power of Spain. But he had also tried to receive a commission from the Portuguese or the French, so this was not necessarily his personal priority.
Third, in 1492 Spain had only just completed the "reconquista", i. e. the conquest of all remaining Arab strongholds on the Spanish mainland. Spain had a strong military force at that time and could look out for new targets now.

Answer 2

"Europe had long enjoyed a safe passage to China and India— sources of valued goods such as silk and spices — under the hegemony of the Mongol Empire (the Pax Mongolica, or "Mongol peace"). With the Fall of Constantinople to the Muslims in 1453, the land route to Asia was no longer an easy route. Portuguese sailors took to traveling south around Africa to get to Asia. Columbus had a different idea. By the 1480s, he had developed a plan to travel to the Indies, then construed roughly as all of south and east Asia, by sailing directly west across the "Ocean Sea," i.e., the Atlantic"

- source: http://en.wikipedia.org/wiki/Christopher_Columbus#Background_to_voyages

Answer 3

Domination of global trade routes.

Answer 4

Money and trade.