Past is past ,is always painful.If past was good then we think why not present is also good.If experiences of past were bad then also it is painful to recall.Most of the people who were good or bad for us in past are not available presently.So it is better to forget the past,except the learning we perceived from past experiences ,being loyal to the persons who were good to us or obliged us in past,few good immortal songs/beautiful natural scenes........