Ostensibly, Hans ' isolation and despair are caused by a stereotypically frigid bourgeois mother, a nagging wife, and a lover's rejection. And despite the complex portrayal--Hans himself doesn't precisely make these claims--the above must be a substantial part of Fassbinder's thinking as well (his use of Freud and Marx). But the viewer may look no further than Hans' gender and sexism to locate the truer cause of his crushed spirit.
First, it's highly unlikely that his mother's lack of love pushed him into signing up with the Foreign Legion. It was far more likely, and is in part indicated, that it was a quest for adventure, male camaraderie (escape from the female world of mother and sister) and male identity itself-- which both the Legion and war offered.
Second, Hans loses his successful job as a policeman because of his own sexism. By falling for a prostitute's wiles at work, he not only rubber stamps prostitution as an oppressive institution, but shows that he cannot even control his sexuality in a professional arena--and is even willing to jeopardize a desirable career.
Third, he commits serious verbal abuse against his wife in front of his sheep-like male buddies, making no distinction at all between her absence or--when she shows up looking for him--presence. In fact, he is more brutal in her presence.
A few hours later, in a violent and drunken state, he beats his wife in front of their daughter, who intervenes on her mother's behalf. The terror he instills in her and his daughter are palpable. But both he--and the audience--move on with nary a whimper of conscience or protest. Why? Because his wife is cruelly characterized as both nagging and sexually promiscuous (yes, this this may be Fassbinder's view of what capitalism does to women--owned, insecure, and a commodity--but this hardly absolves Hans' brutality nor Fassbinder's exploitation of her in the battery scene).
And then there is this male role pressure, which Hans could choose to reject and protest, but instead accepts. He's too short for a male and too un-heroic to achieve the worldly success the male role recommends. But how can these be causes of despair when he not only gains his lost love as a mistress but marries a tall woman who is considerably more attractive than himself., Finally, Hans allows Harry, his war comrade, to remain, over his wife's convincing plea to the contrary, on in their house. By this decision, he not only makes it clear that he is more tied to Harry than to his wife, but that male bonding supersedes his love of women. And supersedes, in the end, his own life, because it is Harry's superior competence and spirit around the house that causes Hans' star to fall. Hans, the merchant, may be to a degree, the victim of capitalism, but more to the point, he is the victim of his own allegiance to his own male identity. His inability to let it go, is the ultimate cause for his isolation and despair.
This is something that is lost, I think, not only on Fassbinder, but also on Han's sister, Anna--although, only to a degree lost. For Anna's (and Fassbinder's) support of her brother--over her mother, only goes so far. She is quite insistent that only he can save himself--that her support and love cannot it itself end his self-loathing. Unfortunately, she does not offer any of this same support and love for his wife who must be much more embittered than Hans but, who in the end, is able to pick up the pieces, and save herself and daughter, and present a marked contrast to Han's fall.