Could a person who isn't depressed take a medication like celexa (citalopram) to feel good?

Depends. Celexa (citalopram) may help relieve anxiety and insomnia in some people who are not depressed.
Depends. All ssri's, including citalopram (celexa), have subtle effects on cognitive function that a person may or may not notice. They reduce obsessive thinking, for example, and they also seem to reduce emotional sensitivity to some extent. Someone who is very sensitive to the criticism of others, or someone who obsesses with worry, may feel better on celexa, even if they are not depressed.
NO. Depression is caused by defiency of " neurochemicals" and if you are not deficient it will not make any difference.