Bad feelings about boys

So I have a situation where every time a boy and I get really close, close enough to almost be dating, I get this weird feeling where I feel like I just want to back out of the "relationship" because it doesn't feel right. Something in my gut just doesn't feel right about saying or getting told the three words "I love you". It really puts me off. I've had this happen to a few guys now and it even happens to guys who I really like. Anyone know why this could be?