If U find yourself in love with someone and that someone does not love U... be gentle to yourself, there is nothing wrong with U, but love just did not choose to rest in that someone's heart.
If U find someone in love with U and U cannot answer that love... feel honoured that love has came by and called on your door. But gentle refuse the feeling U cannot return, as love did not choose to settle in your heart.
If U find yourself in love with someone and the love return... it still can happen that love chooses to leave. Do not try to reclaim it and do not assess any blame. Let it go... despite the pain, there is a reason and meaning to this. U cannot choose love by yourself, LOVE CHOOSES U!!! Peace :D