Why do many people delete all the previous conversations?

1, unwilling to recall. I must have said what I had experienced. )

2. I don't want people who can see it now to see his past (for example, I posted about feelings before, but I don't want people who like it now to see it)

3, delete on the one hand is concerned about the specific object in the previous conversation, on the other hand, it may be deleted in anger. But 2 has a bigger face.