I am sick of hearing complaints. But that doesn't mean I'm unsympathetic, or that I don't think people are entitled to their feelings of frustration.
Venting is fine. Complaining gets old real fast. The difference is motivation: Way I see it, venting is when a person shares feelings they can't bear alone. The venter is releasing emotion. Perhaps they want to be heard because they're struggling and knowing that others are aware of this provides some comfort.
The complainer, on the other hand, is engaging in self-pity. They think they're a victim. Other people need to know because some injustice is being perpetrated. There's a self-righteousness about it. "I don't deserve this."
Venters want, if anything, acknowledgement. Complainers want acquittal. If the complainer's victimhood is confirmed by others then they're blameless. They don't have to own their problems. And they don't have to change.