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俺って誰かさんと違うえらい子だから。Because I am admirable, unlike a certain someone.
空気読んじゃうんだよね。[I] surely can read the mood.
So the POV character is watching two friends flirt and he's pretending to be asleep. When one of them says "I can read the mood", I don't see why he'd use 読んじゃう over 読める for example.
I think there is a sense of doing something without even trying/meaning/thinking. 空気読める is kind of neutral in this regard.
空気読んじゃうんだよね
It's because (without even trying) I end up reading the mood, y'know?
It could also have a negative connotation like "(I don't even want to but / regrettably) I end up reading the mood" which doesn't seem to be the case given the previous clause.
Grammatically
読んじゃう -> 読んで + しまう
