It's a love tap. A swat to the back of the head. Hell, my own parents have done worse on a friendly day. You're creating an act of aggression where there is none. It's like interpreting friendly ribbing as verbal abuse.
Regardless what he feels Marten's motivations should be though it's the same overall issue. Marten's creating a mountain of drama and troubles when all that's happened is that he's been invited to a social outing. He accepted, so he must want to go. Where's this disappointment coming from? Or to use Steve's own words "For some Goddamn reason you're sad about this".
This isn't Steve pressuring Marten to bang everything that presents itself. This is Steve telling Marten to get his head out of his ass and just have fun with a cool situation. Well, okay. It might be the first one as well
. Whatever. What do you expect from Steve?
Seriously though. If I had a friend who would go out, meet someone, get along well with them, end it with an agreement to meet up and have some fun later, and he walks away from this mopey and depressed about it, I'd be concerned. That's not a normal reaction to such a series of events. Hell, agreeing to do something that only makes you feel mopey and depressed is itself concerning behavior. If you didn't want to do this then why did you say yes? Attempting to bring his attention to the fact that he could maybe cheer up a little, that he's doing fine, is the bare minimum that would be expected of me as a friend.