Each of Janie's marriages fail for very different reasons. However, what is far more important than understanding why they fail is understanding the lessons Janie learns from each and how those lessons propel her toward her ultimate goal of true love and self-actualization.
Janie's first marriage to Logan Killicks has nothing to do with love and, therefore, is destined to fail because it will never meet Janie expectations. Logan wants to marry Janie so that he will have help maintaining his farm. Janie's grandmother wants her to marry Logan for the "protection" he can offer her from the evils of an outside world that Janie's grandmother chooses to see. Neither Janie nor Logan is happy in this marriage. Neither lives up to the expectations of the other. From this marriage, Janie learns that love and marriage are not the same thing.
From the experience of her first marriage, then, it is relatively easy for Janie to fall in love with Joe Starks. However, Joe's romantic wooing of Janie quickly fades and, as the year's pass, Joe sees Janie as a symbol of his social standing and his power over others. Here again, Janie's desires are not being met as her thoughts and desires "must be crowded out to make room" for Joe's thoughts and desires. Despite her initial infatuation with Joe, Janie realizes near the end of her many years of marriage to Joe that she will never be allowed to be herself and pursue her dreams as his wife. From this marriage, Janie learns that marriages and love must be equal partnerships between both parties involved.
Finally, then, Janie marries Tea Cake. The "failure" of this marriage is the direct result of Tea Cake's contraction of rabies during the hurricane. Ironically enough, though Janie finally finds true love in this last marriage, it is ended by her own hand when she must defend herself and shoot the rabies-controlled Tea Cake. But from this marriage, Janie learns the true love is an achievable thing and that despite it sudden and sad ending, her life is more full because this marriage and that her memories of Tea Cake and the love that they shared will always be with her.