Why do we find a large number of nude male characters in ancient Greek paintings and sculptures?

Answer 1

The male nude was the picture of idealized humanity. It was not until later that the female form began to be appreciated (and also idealized) in Greco-Roman art.

Answer 2

While certain cultures (Egypt, Persia, Assyria) found nudity shameful, to the Greeks, the human body was heroic. The Greeks preferred showing the human body in its natural state. Further, the Greeks wanted their sculptures and paintings to be disassociated with war. Other cultures portrayed the bodies as victims of war.