Your question isn't silly at all,, it is one that we may have all asked ourselves in one or another. I think that when we make ourselves vulnerable to someone, our "baggage" pops open, and old feelings and issues start to come back,,it is almost like we are looking for familiar territory because we are scared of getting hurt. We have to be aware when this is happening so we can get our perspective on the situation.
I think some people hurt the one's they love because maybe deep down they don't feel they are worthy of their love and subconsciously want to destroy it. I'm not sure, that is only my theory. Either that or they just think that they are "god's gift" and no matter what they do how could anyone stop loving them...of course being a woman I tend to lean towards the belief of the "god's gift" sydrome.