You can't help but fall in love with someone, even though you know that person will never love you back.
He doesn't love you back either loves someone else. The person you are attracted to doesn't feel the same way towards you.
When you are experiencing unrequited love, your judgment may be clouded, and therefore you may not see or perceive things as they really are.
Unrequited love is not a positive experience, so the effect it will have on you will not be great. It can be a source of emotional turmoil.
In the future it is more than likely that you meet someone better, makes you happy, and most importantly, likes you back.
It's so easy to think, "I wish I was a little thinner" or "If I hadn't said that stupid thing," everything would have been different. But remember you shouldn't change yourself for someone else!