Because body, mind and emotions are all linked together. Thus, what you focus on with emotion, you will soon after experience as a form of physical reality.
Try imagining having that person return the longing and then unite with you sharing equil passion. See what happens..
After all, science (though not publicly known yet) has shown that all of life is really a hologram. This means that all you see outside of yourself is truly a reflection of you. Weird right?
Because it hurts to much that you can't have it that you internalize the pain and conflict then end up feeling it for real.
Oh ya, my boyfriend makes me feel that way! And maybe Adam Levine….
The brain is pretty powerful, and the chemicals (beyond hormones) can have very powerful effects on your body. I'm not a scientist, but I'm sure there are very logical reasons that this happens.