Because in spite of common perceptions, expressing negative feelings is not something women often feel they can safely do.
If you’re pissed off, irritated, or annoyed with anything smaller than a genocide, expressing that can very easily get you a reputation as a petty or vicious bitch. You’re told you don’t deserve to even have those feelings, how dare you feel less than amicable about something that wasn’t the result of obvious malice?
If you’re sad, upset, or otherwise feeling vulnerable, expressing that risk earning yourself a reputation as a week, hysterical lady-type who can’t actually take care of her own problems. Just like a girl.
So if someone of any gender is venting to you, or going to you for emotional comfort, they’re probably doing so after having already examined the problem to the best of their abilities. By the time you’re seething with rage, or crying, haven’t you usually put a ton of effort into trying to solve it? Do you feel energized, and like re-hashing ideas you’ve already thought of with another person? Or do you feel like venting?
They’re stressed, they’re probably dealing with some feelings they don’t feel they can safely express, and they’re looking for validation that their feelings are legitimate. They are not going to you to fix a problem. They are trusting you to support them while so that they can deal with the emotional aspects of their problem, which they may not feel safe expressing to someone else.