I think that one of the main reasons I pick is because I have something to do that I'm dreading, and I'm actually procrastinating. I find myself picking when I'm at work, and getting tired of concentrating on work, or I've hit a hard point in my work. Then instead of digging into the problem with more concentration, I dig into my skin instead.
I pick sometimes in the morning before I get up, because I hate getting up, and I dread doing it. Also pick at night in bed, because I'm fighting sleep, or can't sleep. I almost always want to pick if I'm watching TV, unless I'm doing something else also. I have so much to do around my house that I feel overwhelmed with it, and guilty when I'm watching TV. Watching TV is another way that I procrastinate doing something I don't like to do.
The big reward, however, (and I hate to say this, and probably wouldn't except that I've read here that a lot of others also do this) is eating the scabs. I really like that. That sounds awful, but it is true. I wore down my front teeth until I needed veneers, but I still do it now - mostly with my canine teeth (which didn't get the veneers). They are also worn down to the point that they are the same length as the others.
If anyone else has insights as to the reason they do this, let me know.