When you look at people to size them up, you are scanning their energy for weaknesses. What this will manifest as, is you judging them. To do this is violates their psychic space. It is very off-putting. Many times people will think that others don’t like them for no reason. On a subtle level, people know when they are being scanned and will have an aversion to it; which translates to an aversion to the one scanning them.
This is a huge deterrent when looking for a partner.
To break the habit of scanning others, instead of looking for something negative in others, look for something positive. It is much more tolerable to have someone looking for the best in them, instead of the worst.
After you realize that you have indeed been scanning others, and can train yourself to see the best in them; the next step is to pull back and not scan them at all. When you interact with others, it should be in neutral energy. Learn not to step in their energy field, because you don’t want them in yours. You may notice that your interactions with others are much more satisfying.