When you are socializing (one of the first steps in networking), people will judge you very quickly. One of the fastest ways to make a poor impression is to start a conversation with a complaint.
It doesn't matter if you normally don't complain, or that you're having a bad day, or that you're completely right in your compliant. If you're meeting someone for the first time, that person who is the recipient of your negativity will only be able to judge you by that one experience. If you frequently complain about other people to someone, they will soon be wondering if you're complaining about them.
A compliment, on the other hand, is so much more powerful in making a positive impression. It doesn't have to be a compliment of the person you're talking with. You can compliment the location, the event, the crowd, the hostess, anything. It will establish you as a positive person.
How often do you complain compared to complimenting? Are you labeling yourself as a complainer?