I noticed this from one of my female friends. They were the only person I knew who would always say "bye!". Quite frankly, when I see somebody say "bye" I think of it as a very dull voice and that they are bored and aren't doing anything. If somebody says "bye!", my last thought is that they were excited and doing something interesting.

This was something completely subconscious for a long time, but the moment I realized it, it became obvious.