Mitch's tragedy in A Streetcar Named Desire is, first of all, that he is the object of Blanche's affections. This would have been a miserable relationship full of lies and deceit. Secondly, he is an innocent man surrounded by wild friends who try to bring the malice out of him. He is obviously a fish out of water, actually, much like Blanche is herself. Third, he is obviously not the dominant one in any situation: This is obvious in the treatment he gets from Stanley, and in his dependent relationship with him sick mother, to whom he has a huge responsibility.
He breaks down in the end because he also saw Blanche as a form of escape from his dull and unsophisticated life. He said that he had never met anyone like Blanche, and he was indeed impressed. He may have also set store on Blanche as a ticket out of his life, the same way she did with him. In the end, Stanley destroyed both their chances.