I get pangs either when someone I have feeling for (but am not in a relationship with) is close to a female (and I wonder if she's his girlfriend, or going to be, etc) or if I don't know where a partner is and figure he might be with a girlfriend and has not warned me.
In the first case, it's because the guy I'm not with could be mono, therefore if he has a girlfriend or gets one, he won't be available anymore, therefore I'd actually "lose" something (while if it's a current partner, I'm already with him so what do I care?).
In the second case, it's the not knowing part that makes me feel bad, and the idea that he might have gone to see someone else and left me in the dark. It makes me feel neglected and unappreciated. If I receive a text saying he's got a date tonight or something, then that's fine because I was told. Although it's best if he first tells me "I'm thinking of asking X to a date tonight" or "X asked me to a date tonight" so I'm told before the decision is made and not told after the fact.