Its when you have this very good friend. Where you share times together. Where you listen to every word he says. The problems and fears he had to face --- that you are there to comfort him in every way. It is when you give up everything you have. Every dream you have. Just to make him be you first priority. When you give him the material things he had longed to have. When you make love to him knowing that there are no commitments after living the room. When you always listened and read his sms everyday. Not knowing one day he has to leave you for he is now in love with someone else.
Yes it hurts. But you must have to understand that you are just friends. Since the start, now and forever.