Despite the joyousness which is also portrayed, Their Eyes Were Watching God is full of sad and even tragic moments. One of the first such striking episodes happens at the end of Janie's marriage to Logan. When she realizes her dreams of discovering love will not happen, this sad lineis what we hear:
Janie's first dream was dead, so she became a woman.
I agree with copelmat in identifying Joe Stark's demand as being a particular nadir in the novel, but I would go much further in arguing that Janie's hair symbolises much more than just her freedom and individuality and throughout the novel at various stages becomes identified with Janie's non-conventional, defiant spirit.
Firstly, it is symbolic of defiance against conformism, as is shown by the town's criticism of Janie wearing her hair down at her age. Secondly, her hair is described in phallic terms at various points, threatening existing gender relations and challenging patriarchy and this is clearly one of the reasons why Joe Stark orders her to cover her hair. Lastly, it is highly significant that her hair is straight, and thus can be said to signify whiteness - a further demonstration of how Janie's hair disrupts and transcends traditional disempowering binary oppositions such as black/white, male/female.
I think one of the saddest moments in the book is Joe Stark's demand that Janie tie up her hair and cover it with a scarf. Throughout the book Janie's hair a symbol of her freedom and her individuality. Her being forced to cover her hair is a clear indication of the unhappy future of her relationship and the emotional toll it must take on Janie to comply.
A sad moment in "Their Eyes Were Watching God," for me was when Janie realized that she was totally unhappy with Joe Starks. She had run away from a difficult, loveless marriage and believed that Joe was going to be different.
In a way, Joe does provide her with a sense of happiness, material happiness. Joe becomes mayor, he is respected in the town, and is interested in being a man about town, he leaves Janie alone all the time.
Its sad that Janie ends up marrying a second time to find that she is once again oppressed.
Joe Stark's death is a welcome relief to Janie.
I think the saddest moment in the story is in Chapter 19, just after Janie kills Tea Cake to keep him from killing her. Tea Cake was mad with rabies. He rose from the bed where Janie had been caring for him with a pistol in his hand, and "the fiend in him must kill and Janie was the only thing living he saw". She fired at the man she loved a split second before he fired at her, and when he fell "she leaped forward as he crashed forward in her arms". The author describes the next moment as "the meanest moment of eternity". Janie sat with the dying Tea Cake's head in her lap. She "held his head tightly to her breast and wept and thanked him wordlessly for giving her the chance for loving service...she had to hug hum tight for soon he would be gone".