what happens to riley when he leaves????

He goes a little crazy. Rejoins the millitary and marries some woman. That is the short answer. It is in Season 6.

You would find you're answer in Season 6 "As You Were" When Buffy and Riley broke up. As you may know Buffy sacrifice herself to save Dawn's life. She was brought back to life and feels that this hell. Being pulled out of heaven by her friends was the most shocking and hurtful for Buffy to feel. So she want feel something real and need something. So she had the most lustful relationship with the person/vampire that she hates in her life. Spike gave something to Buffy feel again. That it was hurting and realizing something was missing.

After Riley gives Buffy an ultimatum he'll stay if she can care for him but if she doesn't show up before midnight he's going to accept it and go back to the military. They have a mission to South America. Buffy's next love is the love she has for her sister Dawn. Who she protects with her very life and that's her gift!!! When Buffy is resurrected from the dead she finds some comfort in Spike but she knows its not love because Spike is evil!! Later on when Riley comes back Buffy is excited but soon finds out that Riley had married one of the ladies in his unit!!