Jealousy is a bitch. From my past experience, I'm gonna take a stab and say you enjoyed the attention and got on well with the guy. But now he's with someone else you feel down because someone else is getting their attention. It's like, you don't want them, but you also don't want anyone else to have them as it leaves you feeling jealous and betrayed.
If they dumped their gf and came back to you, would you accept them with open arms? I'm guessing you wouldn't, because you'd realise once again that you don't actually fancy them.
Am I anywhere close on this? The frustrations of life eh?!