Because guys are idiots sometimes. Once we have something, we tend to take it for granted. Women do it, too, but I think guys are worse. Then one day we wake up, you're gone, and we think, oh well, I'll just find another girlfriend. And maybe we do, but we can't stop thinking about you. The more we get to know her, the more we miss you, and the more we realize just how good we really had it with you. She's not as caring or sweet or pretty as you, she doesn't have your sense of humor, and she doesn't do that thing in bed that you used to do, you little firecracker you. Finally we can't stand it anymore, so we dump her and come a-knocking on your door again, and we bring a new appreciation of you with us.
You've heard the expression, "You don't know what you've got until it's gone," right? A dude wrote that. I'm certain of it.
Give your boy a second chance. I think you'll find he's a changed man. Learning not to take things for granted is a big part of growing up. Chalk it up to experience and move forward.