For Sam being the "smart one" what he just did was the stupidest thing maybe ever. Well in Dean's eyes it was.

he only caught the last few sentences of the conversation

you were sitting at the kitchen table with Sam, helping him work on the case, you and Dean had been dating for about six months now. Maybe it was Sam's way of trying to start a conversation

but he just started rattling on about how surprising it was that Dean had been in such a long lasting relationship with you.

confused you asked why. And of course Sam told you all about the sleeping around days. The waitresses and all.

Sam even said how surprising it was how he would've went for you because of his usual type

insensitive prick, is what you thought as glassy tears filled your vision and you hurried off to shut yourself up in your room

Dean was livid at Sam. He got an earful before Dean walked off after you