One of three things:
1. He wants a night out with his friends and doesn’t want you around. If so, he’s a big jerk for not just asking, “Hey, is it cool if I go hang with my pals tonight? It’s Mike’s birthday party and he wants it to be just the guys. I’ll be back around 11, and we’ll watch The Daily Show together and bang.”
2. He is mad about something, and keeping it to himself for some reason. Going to a party and not inviting you is his passive aggressive way
of showing you that he’s pissed.
3. He wants to flirt with other girls and/or cheat on you.
And that’s….pretty much it. Either way, he should explain himself. If he wants to go by himself, he should tell you why. Plain and simple. Doing stuff separately is perfectly legit; leaving you home alone with no explanation is not.
Confront him on it. Don’t let him get away with it. And then go to a raging party on a Friday night and leave him behind. That’ll change his behavior right quick.