@sats,
Quote: why does he specifically keep coming in and out of my life,
Based on what you wrote, he is not continually coming in and out of your life. You knew him in high school, he came onto you, you rejected his advances, he stopped talking to you, and now, 3 years after HS, he is attempting to contact you again.
Basically, you are simply wondering why he is wanting to contact you again. Well, either he wants to reconnect with someone he knew in school simply for the friendship or he wants to try again to get in your pants. You won't know until you start talking to him.
So the question you should be asking is not what his motive is for wanting to reconnect, but rather if you want to reconnect with him? Only you can answer that one.