One of the main reasons a person belittles another is simply because they see many traits of their own in the other person. This is something that they can't handle. This is because although they do not recognize the fact that they themselves are what they are seeing in someone else. To them they are just not recognizing what they don't like about the person as one who is displaying the same traits as they possess.
We all at one time or another put someone else down, because at one time or another, we did the same thing, and just because we are way past that particular point, we now are experts in seeing our behaviour that we once had ourselves in someone else. Take the new guy in a bar scene. He doesn't have the experience of a seasoned bar hopper in the way of picking up a girl, so his mistakes are pretty obvious.
Now anyone who may have been there, done that, will find it amusing, and use it as a chance to put down the poor sap who is just trying to find his way in the single world. Another example would be a contestant on American Idol making it to the top. The professionals who worked hard to get where they are, are quick to put the winners of American Idol down. It's like unless they've paid their dues all along, they are not worthy of the fame they attain.
So it's the "I'm a seasoned star, what are you" type of attitude. All the mistakes in life that we make will at one time or another give us an excuse to judge or belittle others. Not all of us, but a great deal will fall into this category. We can spot when a person is nervous because we at any given time have the same look about ourselves when we are nervous.
We can spot when a person is trying to hide their feelings, and the reason is, we do it ourselves now and then. So it is what we don't care about ourselves that we are in fact guilty of that we so easily see in other people. When those who belittle others learn to relax with what they see in others that cause them to belittle another, they might just find themselves a little more secure about their own personality.
Life is a learning experience, and sooner or later, we all learn what we need to know to feel at ease with who we are.