Why does a guy have an erection?

Answer 1

Erections make it possible for guys to insert their penises into vaginas during intercourse. Stimulation of an erect p***s leads to ejaculation, which is the means for getting s***n and sperm into the v****a, which in turn could lead to pregnancy.

Guys get erections for many different reasons:
  * Sexual arousal
  * Physical stimulation of the p***s
  * Tight clothing
  * A full bladder
  * While asleep
  * Sometimes for no apparent reason at all.