Along with his playfulness and innocence, Joe has an incredible devotion to Missie, like a puppy to his new master or a child to its loving parent. It is important to contrast his feelings and his approach to her with Slemmons who appears to be worldly and knowledgeable and sophisticated.
Instead of sophistication, Joe has this dedication and incredible love for Missie that includes the ability to forgive her for her mistakes and her affair with Slemmons.
While Slemmons maintains an image of wealth and privilege, Joe is actually just as devoted and loving as he makes himself out to be. There is no deception in his playful and powerful love for Missie. He would honestly have all the women in the world die rather than Missie suffer the slightest ailment because she means everything to him. He is completely willing to dedicate his life to her and keeping her happy, he could care less what other people think of him or how they might view this devotion of his.