What do you learn about friendship in Julius Caesar by Shakespear?

The Lesson to be learned concerning friendship with Brutus and Caesar concerns loyalty being a two way street. Caesar took all the power for himself, rather than share some of it with Brutus. This was a sort of disloyalty towards Brutus that resulted in the retaliatory backstabbing. It is a lesson of the delicate balance necessary when mixing friendship with power structure. (Don't mix work with pleasure.)