Goin' have to agree with the last two comments, Finn's facial expression is truly priceless and the very second thing I noticed.. after Marceline flexing of course. Got to say his slumped pose flows nicely with his face, great body language. Of course the same can be said for M and B as well.
Jake can feel the love in the air ... I think he's more okay with people in love and stuff. Finn feels like a third wheel I think. Either that, or it's before he's meet his true burning love Flame Princess :3