Again, I’m not sure why guys complain, because I’m not a guy. But for girls, they don’t complain because they want you to solve their problems. All they want to do is talk.. I’m serious about this.. sometimes they don’t even care about any special form of reply. There’s no need to analyze what they’re talking about. They don’t want you to do anything and they’re not telling you their problems because they think there’s something you can do about it. There are a zillion things you can reply them with, something as trivial as “oh, go eat some candies” would do just fine. What they don’t want to hear is “don’t complain to me, because there’s nothing I can do about it.” You think they don’t know that?
And seriously, there’s no such thing as a girl who doesn’t complain about anything. Even if she’s independent enough, she’ll still complain to you because naturally she just wants someone to listen to her while she’s coping with her problems. There’s really no need to get pissed/upset/emo about the fact that you can’t do anything to help her out. Have a little faith in her and believe that things will be better for her sooner or later.