We criticize because we somehow feel devalued by the behavior or attitude. Critical people tend to be easily insulted and especially in need of ego defense. Critical people were often criticized in early childhood by caretakers, siblings, or peers, at an age when criticism can be especially painful.

What does it mean when someone constantly criticizes you?

A person being constantly criticised is likely to find it hurtful and demoralising and may grow to resent the person doing the criticising. If you find criticism has become an issue in your relationship, it's important to nip it in the bud before the problem becomes any worse.

What do you call someone who constantly criticizes?

Probably a "hypercritic" - a person who is excessively or captiously critical. Or "hypercritical" as an adjective. You could concatenate "constantly ratifying", "self-righteous" and "hypercritic" to call such a person a "constantly ratifying self-righteous hypercritic".