Something was bothering Sunstreaker, it was easy to tell even if he didn't say it

He clearly shows it through his actions and facial expressions, like when had he ever hidden it?

Definitely not when he's with you

It's a mystery of the universe what made the two of you become friends, but it already happened. It safe to say that friendship between you and him is genuine and real, what they call it again bestie? or something around it

Sunstreaker can prove himself to be an amazing friend and a pain in the ass to deal with and now he's the latter, Acting picky and much worse than his usual self, especially when it comes to your new friend

“Oh, you just said you knock that con out? think I can do better than you” all bluffing and cocky with that smug on his face as he invites himself into the conversation between you and this new friend of your