Question

why did nick break up with marjorie in "the end of something"

Answer 1

Nick tells Marjorie that it is not fun anymore to be together so he wants to end the relation and break up. Even though he feels bad afterwards on breaking up with Marjorie and have no friend, but he tells Bill that it was easy and he is fine. 

Answer 2

Nick tells Marjorie, after a picnic supper that he wants to break off their relationship. He says that he felt that it was no longer fun to spend time with her. After she goes away Nick feels bad about breaking up the friendship. But he tells Bill that the breakup was not very difficult.