Shame, Fear, Guilt, Assumption nothing will get done, or not wanting to delve into the memory - any of these keep people silent when they should not be.

I've known people to admit to not wanting to sleep with someone, but doing it because they feel expected, didn't want to damage a friendship, or otherwise - even known the partner would stop.

People are rarely honest about Love, love, or lust. Those that are tend to scare away others who can't be for whatever reason.