Very loosely inspired by the "irrational things that annoy you" thread.
Growing up, at many of the schools I have attended, you just could never be late. You'd get detention or a severe or embarrassing (amusing for everyone else) telling-off.
Maybe it's because of this, but I hate being late. If there's a chance that starting something else could potentially cause me to be late then I just don't do it.
Of course, I still am late sometimes...