Pride, then, seems to be a sort of crown of the virtues; for it makes them greater, and it is not found without them.
Aristotle, Nicomachean Ethics, c. 350 B.C.

Pride is the virtue of respecting oneself. It is a human need to think highly of oneself. Without it, one would have no reason to trust one's ability to live. One would have no reason to accept that one's life is worthy of living.

Pride is often confused with arrogance. Both seem to evaluate oneself highly. The difference is fundamental, though. Pride is a rational evaluation of oneself. Arrogance is not. Pride requires one to think highly of one's accomplishments and abilities. But the accomplishments and abilities need to be worthy of the praise. Without them matching, the false pride will lead to self-hate when reality undermines the attempted illusion. If one's abilities are not as good as one would like to pretend, it is just a matter of time before they are genuinely tested, and the results will destroy the flimsy self-esteem.

True pride, on the other hand, is rational. It has the secondary consequence of making an person want to improve himself in order to feel greater pride. This secondary effect, though, is not the reason for accepting pride as a virtue. Pride is virtuous because one needs it to live. It is the pillar the supports one's mind. Without it, one would constantly question one's ability to make rational judgments. It would undercut reason, man's primary means of survival.