It's "you" so no one outside you can prove you to you you just tell a doctor/therapist your thoughts, feelings, behaviors, etc. and they have a professional opinion on what treatment may help you manage or fix any troublesome thoughts, feelings, or behaviors.

A bouncing leg can be all sorts of things, a problem to you or not, and isn't really indicative of anything, for example. It is everything you can possibly tell a professional that bothers you (or you have heard bothers others; teachers, parents, etc. complaining) that might look like a pattern. So, listing your interests over time helps them make up their minds whereas picking one thing or another, no matter how extreme the behavior, cannot give as accurate a picture.

It's the principal of: All that glitters is gold, glass glitters, therefore glass is gold. People with schizophrenia sometimes talk to themselves, I sometimes talk to myself, therefore I am schizophrenic It doesn't work that way.