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.
However, when one day you realise the one you love, loves you back, it gives you the most amazing, elated feeling you never thought was possible.
1. Romeo to Benvolio - "well, in that hit you miss: she'll not be hit With Cupid's arrow, she hath Dian's wit;"
2. Tom will never know how much he means to me... after all this time.
Even songs are written about it: "And behind his tierd eyes, she sees the boy with his arms wide, who made her feel like an angel... 'cus she's lovin' him still for the rest of her life..."