I am an avid supporter of complaining. I hate when people try to have me 'see the bright side' while I'm complaining about something. In it's proper place, complaining can be a useful tool to improve things for the better, or at least get some sympathy. Sometimes, sympathy is all you really need.
Many times, motive to improve my lot, comes from complaining about it. If there's nothing to complain about, then there's nothing that needs to change for the better. I just don't believe that absolutely everything in my life is at the standard I want to reasonably achieve. On the other side, if there's something lousy in my life I can't change, I should not complain about it. There is no point to that.
What I do hate, is a complaining spirit. There is nothing more foolish than complaining about things, for the sole purpose of being dissatisfied. There is no improvement here.
I think people with a complaining spirit, never complain to anyone that has the power to change it, or the desire to feel sorry for them. They just complain to people that can't change it, or don't care about them, so there can be more complainers, saying:
"Hey, you're right, they really should do blah blah blah, instead of blah blah blah"
Whine whine whine.
I would say the majority of my complaining is simply a request for sympathy. A little bit of it is me trying to become more dissatisfied with things so I get motivated to actually change it.
I'm glad I thought about this, because there are some things I need to change about myself. I should not assume that someone who complains to me is just a complainer. I'm going to make an effort to give them sympathy.
Usually, my first reaction is to do the exact thing I hate people doing to me: making them look on the 'bright side'.
Plus, if they are not just requesting sympathy, it will make them stop complaining, because that's not the response they want. I think sympathizing with a complainer has a potential personal entertainment value as well.
So complaining can be a useful tool, if you view it as such. Just be consistent in your view of it, when dealing with others.