If Adam wasn't looking for a long-term relationship, then why go on the show and state that that is what you are looking for?Adam was pulled into the trap that we all have probably fallen into. He was enamored of Samantha and the "passion" (read lust) that he felt when he was with her. The adrenaline rush that accompanies new love (read lust) is hard to overcome, as Adam demonstrated repeatedly with his breaking of the vow not to kiss each and every girl. As my wife said, once they had sex a few times, I'm sure that Adam realized how shallow Samantha was.
The passion that Adam supposedly was missing with Rachel was there, but I think Rachel was smart. It was "missing" b/c she restrained herself. She was there for the right reasons and didn't want to come across as a ho-bag. In the end, she lost Adam but retained her dignity. I only wish she had given him his bracelet back.
On another note, I think my favorite, Amy, also scared Adam off with her questions about geographic location and religion. Those questions showed that she was serious about the relationship and really threw Adam off, I think. Of course, that didn't stop him from making out with her and putting cake in her cleavage, but, hey, he was living in the moment, right?! All fun and games.