Why does Huck reveal the plot to Mary Jane?

Later that morning, Huck sees Mary Jane sitting on her floor, crying while packing to go to England with her uncles. Mary Jane explains that she is upset about the slaves being so mistreated. Huck feels sorry for her and is also attracted to her. Huck blurts out that they will be together again in two weeks at the most, knowing the Duke and King will abandon the town. When he realizes he has slipped, he decides to tell her everything.