I am amazed at all you people saying it's too hard to express an answer to this simple question.
Why do you love a person and be in love with that person.
1. It's about feeling "whole" with the person you love. Every time you think of her / him or alone without her/him besides you !, you feel you are half empty ! Only to feel FILLED when you are with her / him
2. Because there's someone you can always talk too, cry with, laugh with, share you feelings with and at the end to feel all "OK"
3. Because with the person you love, you can fight with him / her, argue with, be annoyed at each other for sometimes but at the end, you are still deeply attached to that special person and cannot spend a second without her / him
4. Because you have a shoulder to rest you head !, This world can get cruel sometimes and things can make you feel really down and bad. In all those matters, may be you are right, may be you are wrong. BUT you know in your heart that what ever the side you are, there's a person who truly believes in your side and will stand by you for it!
5. And best of ALL !
You don't have to wear expensive colonges and cloths to make him/her impress anymore. You know that even if you just show up with wrinkled cloths and smelling like a pig Stine, there's a person who is always there to give you a hug !