Why are feelings "wrong" when they are not reciprocated?

Do some folks not understand that affections are an integral part of human nature? Who are they or who do they think they are to decide that somebody's feelings are "invalid" just because the person being crushed on has a significant other? What makes them feel that their relationship with their significant other can last forever either? Are these simply narcissists who see legitimacy in their own perceptions of every single matter?

Hmmm so feelings are okay/valid if they are yours ("Who are they or who do they think they are to decide that somebody's feelings are "invalid"), but not okay/valid for someone else if you dont like them ("What makes them feel that their relationship with their significant other can last forever either"). You cant have things one way, if your feelings are valid even if people disagree with them, then conversely so are theirs even if you disagree with those also.
You are allowed to be angry and upset about what these other people said, but that doesnt mean other people need to feel the same way.
Greg

You can have whatever feelings you wish, the problem is when you act upon them in an inappropriate way

Define inappropriate. I want to know what prerogative you have to decide what is inappropriate.

Annoying someone who doesn’t share your feelings

Define annoying? What if the person keeps it to themselves?

They have the right not to feel the same way, but they don't have the right to invalidate my feelings - not even if they are narcissists seeing legitimacy in only their feelings.

They actually do have that right, they are under no obligation to value or validate your feelings and there is nothing you can effectively do about it apart from remove yourself from the interaction.

They "have" the right only when they are narcissists

They "have" the right only when they are narcissists