The baby is adorable. Dean does not feel his manhood is threatened by admitting that to himself. The fact that Sam holding the baby makes him want to jump Sam and do unspeakably dirty things to him, however, is making him twitch a little. Damn it, he's not a fucking girl.
Which reminds him, girl. He's really pissed off at his brother. Yes, right. Angry. He's angry with Sam. Dean finds that, normally, if he keeps reminding himself that he's mad at Sam, he's better at, you know, acting on it before Sam has a chance to make the puppy eyes, which can only lead to more unspeakable badness. Right. Because, a year-and-a-half ago, he's pretty sure that he and Sam had that fight. That fight. With the punching and the leaving. Yeah, he's still angry about that – there, anger, at Sammy. Sammy who went out and fucked a girl who then got burnt up on her ceiling and then a fucking baby arrived on their doorstep with Bobby. Right, angry with Sammy.
Dean can do that. Easily. Except, yeah, there are the puppy eyes, and the pout, and Sam is tickling the baby. Dean's pretty sure this is exactly how they ended up carting around a kitten until it 'escaped' (or got given to the Humane Society, Dad never told and Sam would probably seethe and sulk for weeks if Dean told him the truth) when Sam was five. Dean is, in fact, already planning on asking Bobby if he'd mind them sticking around at his place a few weeks, because he knows, in the small, practical part of his brain usually used for hunting, that the baby is staying, except this time he'll get in serious shit if he gives it away.