Sam is trying to establish his independence as an adult. Yes, he's going out and doing things on his own without Dean's permission or approval, but Sam is 26; he doesn't need Dean's permission or approval, but Dean is still trying to maintain that parenting role to his little brother he's had since he was 4 and have some control over Sam, which Sam is chafing at like mad.

Sam and Dean have already talked about Sam using his powers to defeat Lilith and Sam knows that Dean is totally against it, so at this point he's just trying to avoid constantly provoking confrontations with the person he lives with 24/7 that end with him getting punched in the face repeatedly. I can't really say I blame him.