That's because of the fact that They were interested in that one person. It matters not that there are others. It is like an addiction. An obsession. Their world begins and ends with that person. That person is the reason they are living. But when that person is not available or that person rejects him/her, it is damaging to the seeker's psyche. Any number of things could occur in the mind of the seeker. And the object of their affection has no idea how their response will be received.
you are so right about that I nicely but firmly rejected a ex girlfriend advance and after that she came after me with a Butter knife. I personally think sometimes females handle rejection a little worse than males