Why do we keep coming back to each other?

This guy and I have had this casual thing for years. But it always ends the same, life gets in the way and we take some time apart and even though we have both said in the past that's it now, we always end up coming back to each other. I don't know why? It's like gravity we just end up drawn to each other. I don't know if it's a comfort thing but when we are together it's like the time we where apart was nothing at all and nothing has changed.
Why do we keep coming back to each other?
Post Opinion