Viral Emotion?

I've been wondering about something that came up during my ASD assessment. 

Anytime anyone in my life, close family member or acquaintance, is really happy about something (new job, new baby, long-awaited and much anticipated purchases arriving, mastering a new skill ... just anything at all that would cause a person to feel happy) I can say with some confidence that I feel happy for that person. Note: FOR them.

Apparently this is a classic sign of ASD, because I don't actually feel the exact same happiness INSIDE me as I would if it were me doing / getting / experiencing these things. Apparently NT's actually feel the exact same happiness inside themselves for another person's happiness? As if happiness is a transferable thing, like a virus? This is mind-blowing to me! (If it is indeed true.) I've since read that this is true of ALL emotions as experienced by NT's, which sounds ridiculous and exhausting to me! 

Fear? Confusion? Worry? Excitement? What possible good could it do to simply replicate the other person's feelings? And if this is all true, then how is unrequited love even possible? Wouldn't feeling love for someone automatically make them feel love in return? Clearly that doesn't happen so what is going on here? Do NT's feel the other person's emotion then decide whether or not to ignore it? I can't believe that this viral emotion actually exists, it sounds more like a design flaw than a desirable trait. 

Has anyone else encountered this weird theory? What am I missing here?