Its the feeling of being completely, hopelessly, desperately in love with someone, all the while knowing that your feelings will never reach them.
Its contradictory in that you feel incredible because you love someone so much, but also at the same time you feel almost overwhelming despair because you will never know what it is like to hold them in your arms, or touch their face, or kiss their lips. You will never know what it is like to wake-up next to them in bed in the morning, bodies entwined.
It can be masochistic in nature- it causes such heartache, but you enjoy loving the person so much, you willingly let your heart go through the agony, so you can hold onto something- I don't know, maybe hope?
In any case, unrequited love is a very sad thing.
Its like drowning but you just won't fucking die.