Humans like to categorize things. They like to label and organize, in hopes they'll understand and have some control. The only trouble is a lot of things can't be labeled or even understood. We have a lot less control than we imagine, and that's not a bad thing. When we have no control we're forced to trust something bigger than ourselves. When we don't understand we're nudged into learning about the subject we question.
One of the things humans like to categorize is other humans. That means you. We like to paste on labels like "teacher", "lawyer", "sewage plant worker" as if that can sum a person up. I don't know what we do when someone changes jobs.
Men have a harder time with career as identity than women. This can leave them directionless and empty when they lose a job. For those without careers, their hobby is their identity. "Todd is a Reds fan. He's always sporting team colors during the season. Frank is a downhill skier. He won gold in the last olympics."
But what happens if Todd stops watching baseball or Frank is injured and can't skii anymore? They don't simply disappear or shrivel up, but they might feel like it. They lose confidence and begin searching for a new identity, something to latch onto that will say "This is who I am."
What keeps me determined when I feel I'm being stuffed into boxes is a fluid identity. I know I'm not just an artist, writer, library clerk, art teacher, blogger or whatever I happen to be working as. Each of those are things I do, but they all spring from the same source. It's creative, adaptive, playful, imaginative, childlike, intelligent, patient, and sharing. No matter what my job is, it will involve those traits. As long as I connect to that source I will never be unhappy.
This is why individuals shouldn't put themselves in boxes. Society will always categorize, label, and cattle-prod you into a little cube of what it thinks you are, but you were born to be fluid, to be adaptive. As long as you keep in touch with the source of who you are and find a way to express it, no matter what your job is, you'll be happy.
So what're you waiting for?