THOR: We must do this! You’re not going to let my brother and me take all the glory are you?
THOR: What you are coming with me aren’t you?
LOKI: Yes of course! I won’t let my brother march into Jotunheim alone.
He’s so surprised Thor wants him along… and then he’s so proud to be included, even if he knows it’s a terrible idea and is going to do everything he can to keep them from getting to the planet and messing things up.