I used to love a boy deeply.For this,I as well as him were hurt.After separate while I met him by accident,I realized that I have forgotten him.What cause this? Maybe after lovers separate that they can't be friend.I feel sad and I tried to make us be friends,but I failed.I need your advices.

Ah, Sweetheart, this is one of the hardest things about becoming an adult. Sometimes, even when we love someone deeply, it just is not possible to have a relationship. Sometimes it's because all the love in the world can't make two people fit together in a way that lasts, sometimes it's because each person has goals or priorities that are too different and, sadly, sometimes there just is no reason, it just isn't quite right.
Go ahead and feel sad for a little while and then cherish the memories and when the time is right, you will meet the person who is also right.