There are three reasons some guy in Philly you hooked up with just isn't returned the love. Don't worry! I'm here to outline the reasons and tell you what you can do about them.
Reason 1: He's just looking to have a one-night stand. Personally I think these are pointless and not even that enjoyable but some people love them. There's nothing you can do about these guys. They have made up their mind about their intentions with you before you even started making out. You can defend against these guys by not letting them over or going to their place for a while and see if they stick around or not.
Reason 2: One everything was done he decided your personality wasn't the right match for him. Sometimes dudes aren't thinking clearly pre-hookup. We ignore obviously red flags because of a physical attraction. These guys are tougher to defend against because they genuinely believe they like you until it goes down. Just know that there's nothing you could have done differently and try to move on.
Reason 3: This is going to be the toughest one to hear - you just weren't good/compatible in the bedroom. This is tough because no guy will EVER tell you about this, most of them will just act like it's something else. For some reason men have a difficult time saying anything about an incompatibility/issue. If you find that hookups with no follow-up keep happening to you this might be something to investigate. Really ask for some feedback next time you are with a person that you trust to give you the hard truth.
Reason 4 (bonus reason): He sucks.