My girlfriend is watching a movie so I think I'm gonna go in there and start playing my guitar.
I don't thinks he will like that. Why do you want to bother her?
I'm gonna do what I want. I don't care what she's doing.
Your girlfriend wants to focus hard on the movie, and it's rude that you keep wanting to be selfish and noisy, disrupting her. You need to respect your partner and not disturb them or else they will be annoyed with you.
I'm done. You sound like my mom.
Your mom would honestly probably also say the same thing. When your girlfriend is trying to watch a movie, you should be quiet so she can enjoy it. Instead of disturbing her, you should sit down next to her and watch the movie with her!