[peter saw damon and fitz's conversation on... klaus' network post (yeah, that's the guy peter invited into his room back after faith died without a second thought. great.) and he knows what fitz said wasn't true. it was peter that told him about damon in an attempt to connect with him about the compulsion that he'd gone through. he's really grateful he ended up talking to elena and bonnie beforehand or else his relationships with both of them could have seriously been damaged. it's not like his friendship with elena isn't already. she's understandably angry, and he's going to try to make it up to her, but their friendship took a hit for it. he feels guilty that the words just stumbled out of him without thought, and he's going to try to be more careful now.

it's just not a world he's used to living in where secrecy is that important. no one ever really suspects that he's spider-man. he doesn't have friends so it's not like he's had to try hard to keep secrets from anyone but his aunt.]

I didn't tell you that Damon was a vampire so you could inform him that you knew that he was a vampire the first time you talked to him.