Why can I not pee when I'm in a public bathroom?

Stage fright. It happens to the best of us.
Paruresis. Paruresis is the avoidance of, or the inability to use the toilet in the presence of others. It is listed in the dsm-iv, the classification manual used by psychiatrists, as a social phobia. This can be a serious condition & an issue in cases of employer-based urine drug screening. For those who experience significant anxiety, treatments such as cognitive-behavioral therapy & support groups exist.