Why People Complain?

I think it's natural for people to complain.
Everyday we're faced with a lot of reasons to complain:
we're not tall enough, we don't have the latest and greatest, it's too hot, it's too cold, they get everything handed to them, they shouldn't be doing this they should be doing that, etc.
Having the occasional complaint is fine. To expect a life without complaining (by you or others) is unrealistic.
Why we complain ?

Life's not fair. I think most of us complain because we feel things are unfair and out of our control. Why do they have so much when I have nothing? I've worked just as hard, saved harder and I still don't have as much to show for it. It's just not fair.

Genuinely unhappy.
Some people are just genuinely unhappy. They can't find joy in any aspect of their life. Genuinely unhappy people take to complaining to express their dissatisfaction with their life and in many situations it's used as a coping mechanism as well.
Jealousy
Most times people won't admit to this as it's a rather unbecoming trait. They may also not admit to it because they truly don't recognize it as jealousy. This ties in with the first point of life not being fair. The emotion that can come out of that is jealousy. When you wish you had the new car, or the new job or the high salary there is a tendency to complain, either that the other people are getting this or that you're not.
Lack of empathy.
A lot of times we complain because we lack empathy for others. Think about this: You   complain that someone isn't doing something "right", they should be doing it this way because that's what we would do if we were in that position. When we lack empathy we aren't able to fully put ourself in the other persons place. We don't know the thoughts going through their head or what other complexities they are facing in their lives at that moment.
What can we do about it?
Think before we speak.
Before blurting out a complaint really give some thought to what you're going to say. Is it a complaint for the sake of complaining? Are you really unhappy with your situation? Is there any benefit to expressing this view point? If you are looking for insight from the person you're speaking with then by all means go for it. If you're mentioning it to spark a debate or initiate a conversation, OK. Just be aware of what you're saying and why you're saying it and also be mindful of the person you're speaking with. Are they rolling their eyes? ;)
Have a plan.
I find it quite irritating when people complain about their job or people they work with or how they're bored with whatever it is they're doing at the moment. I find it even more irritating when people have no answer to the question "and what are you doing about it?" Some people have given it some thought and others just stare blankly as if to say "There's something I can do?" If you're unhappy with any situation make a plan on how you can change it. How are you going to take control and make your situation better? You can brush up your resume and start applying to new jobs. Look at how you can possibly move within the company you're in now. Can you go back to school or get specific training for what you'd like to do?
Don't just moan about it do something!
Change your perspective. 
Maybe you're complaining because you lack empathy for someone. Change your perspective and really put yourself in their shoes (and not just in one particular situation, but overall). Your thoughts, expectations and judgments may change and you may see the other person in a better light and not complain so much about how they're handling things.
A change in perspective could also be helpful when we're complaining about what we don't have or how we don't measure up. First of all we shouldn't spend so much time comparing ourselves to others in the first place but if you do why is always with people who have more? Why not compare with those who aren't as fortunate as you. Look around you and really see what you have. There are a lot of people who would love to be in your position. Which leads me to my final point ...
Be grateful. 
Be grateful for what you have and for what others have. Life isn't a race to see who can collect the most money, cars or other shiny objects. Ever told anyone to mind their own business? Well same goes for you. Focus on you and what makes you happy and how you can add value to your own life. Show appreciation for all that you currently have and for what you've been blessed with. Don't worry about all the other people odds are they're looking at you and wishing for something you have and that you may even be taking for granted.
When we complain it's usually because we're so focused on what we're lacking in certain aspects of our lives. The easiest way to curb our own complaining is to focus on the positive and go with the flow.
Do you have any other ideas on why we tend to complain and what we can do about it to make it stop?