Dean is oblivious to a lot of things, mostly because he's stressed out every single day. As his little sibling, you'd gotten used to it.
But, you've wanted to tell him something you've been hiding for so long that he didn't seem to notice. You didn't feel comfortable as the person you once were, you've changed. Dean shrugged it off as "teenage hormones", saying he went through changes too, but there were huge differences between him as a teenager and you.
Dean only started to notice things when you started acting differently. Too different. You would wear more masculine clothes that hid your frame, which he was totally fine with, but something was off the moment you came out of Bobby's bathroom, hair cut off. It was short now, extremely short.
"Kid, you know I'm not some kind of authoritative person like Sammy," Dean spoke, having you sit down on the couch while he spoke. "But mind telling me why did you cut your hair? Specifically, why so short?"
Dean had an idea of why you did it, even if he was old fashioned. But he'd rather hear you trust him enough to tell him.