Why is Joe also running away?

Joe wanted to run away with Tom because he didn't think his mother wanted him around anymore.

His mother had whipped him for drinking some cream which he had never tasted and knew nothing about; it was plain that she was tired of him and wished him to go; if she felt that way, there was nothing for him to do but succumb; he hoped she would be happy, and never regret having driven her poor boy out into the unfeeling world to suffer and die.