If you aren't related to them, they will get depressed. As they know, they'll never date you, or anybody else.
Even if they are dating you, they'll still get depressed. You'll have to ask a psycho doctor why.
Don't do anything that could be considered affection by an insane man either.
So, if you are a doctor, don't give an insane man a prostate exam, they might consider that love.