why human just fall in love.

because it is human to fall inlove. Man live because of love and die all because of love.

Honestly, I think the problem arises mainly when 1) someone is too quick to declare "love" in response to the other party in order to avoid hurting their feelings or some other ridiculous response or 2) when one person does not understand what love is and thinks the passing of those lovely initial hormonal surges means the end of love. I haven't been in your shoes, but my poor friend was and I had to tell him the same thing: you can't know what she was thinking or feeling, or the motives for her actions. It hurts now, but you can find true love someday, probably when you're not looking. Sois fort - be strong. Good luck.