Candide

Why do you think pangloss constantly says "its all for the best", What reasons does he have?

In my opinion, I think he constantly repeats his slogan 'Eveything for the best' because he is optimistic and by thinking optimistically, the world will look like a better place. 

Pangloss says that everything is for the best because he is convinced that events take place for a reason, and that reason is called fate. he believes that if something bad happens it is all for the best, he won't try to stop that bad thing even if he had a chance to. he thinks that would mean change destiny's course.