Answer: Buddy arrived whenever someone was sad or lonely. He was there to help out, and when the person had gotten through their sadness, and seemed to be doing well, he moved on to a different human, who needed to be cheered up at that time.
Buddy wants to do more than just exist with Ethan: he wants to better Ethan's life. So he concocts a scheme to get Ethan back with his childhood sweetheart, helping Ethan find the happiness he never had.
