You've surely hung out with an intro psych student at some time in your life. One day you're having a regular conversation with them, and mention something vaguely abnormal you were considering a few days prior. They perk right up, look at you intently, and start asking questions about why you have that particlular idea. They also ask questions about other stuff seemingly at random, like your sleep habits, what kind of jokes you find funny, your shoe size, etc. After about fifteen minutes of this activity -- which seems to actively take up a great deal of their mental processing power -- they pronounce a verdict about your mental health (or lack thereof).

You are bipolar.
or
You have panic disorder.
or maybe
You should to be taking SSRIs.
and so forth...

I myself can't claim innocence on this front, as I'm in an introductory psychology class right now. Instead of "diagnosing" other people, though, I tend to find aspects of whatever the day's lecture is about in myself. Did I once have an eating disorder for six months? Are my screwed up emotions a function of very slight autism? Do I twitch because I have an anxiety disorder? Does thinking about all of these things mean I'm paranoid? I know from talking to others in my class that I'm not the only one that participates in this variation, either.