Socialize - it's hard to go around and say hello to friends since I'll be worried she'll be bored.
Drink - I have high stamina when it comes to drinking, she tends to be sleepy and tired easily. I don't want to feel guilty gulping some beers while she's trying her best to stay awake.
Trashtalk - let's admit, there are some sensitive topics we like to share with close friends wherein we don't want our SO's to hear about it.
It's not me being selfish if I want to go alone, I would just like to talk to friends with ease. Given my attention span, I might have a hard time juggling my focus between my SO and friends. But if it's a small crowd, won't have doubts to take her.