This bothers me as well. It usually comes right after I tell someone that I am bipolar. People say "Oh I could never tell that you are" or "I'm probably bipolar too".
1. you don't know what is going on in my head as I try to hide what's going on in my mind at times. The people closest to me are the people who see what I am *really* like at times.
2. I know that you are trying to relate to me by saying "I'm probably bipolar too" but it's different. It's an illness, not just mood swings, getting angry at times, or having a bad day.

It can definitely be irritating. Especially the one day, my boss came up to me out of nowhere and says, "I know you're bipolar, but your's isn't that bad... I think so and so is bipolar. You're pretty stable.". (her ex was diagnosed with bipolar so she does know about the illness, however, each person is different and again, she's someone who is not with me outside of work. When she made the "stable" comment, I was actually doing a lot of cycling.)

I try to just brush off people's comments because I feel that those who make comments like that don't always know what they're talking about. I think if people were more educated about the disorder then they wouldn't be so quick to say that they self-diagnose. I saw that one of you said you were recently diagnosed, and this is in no way meant to offend. Welcome to the club, so to speak.