Excuse me, but how is he selfish in either story or movie?
Clearly in some ways he would much prefer the sweet life with Jack over what he's "stuck with" in Riverton. His choice to stand it could fairly be described as mistaken, shortsighted, deluded, conformist, paranoid ... all kinds of negative things. But IMO it doesn't seem the least bit selfish.
I consider him very selfish. He's a poor husband to Alma. Just because he's not in love with her doesn't mean he should sit around home and mope and never take her out anywhere. Jack is clearly a social creature and steps out with his wife regardless of whether he actually likes it or not. It's for
her you see and not just for him.
Ennis never brings home any fish for his family. Ever. Never even thinks to stop and buy some.
Ennis never offers to come down to Texas, save Jack a trip. Jack always has to come to him.
Jack drives all the way up from Texas upon word of Ennis' divorce. Ennis has the kids that weekend and doesn't even offer Jack the opportunity to stick around so he could see him later. He just lets Jack drive all the way back.
Their relationship was always on Ennis' terms. Always out in the country, in the cold weather, when he knows Jack is always cold. The time Jack drives up to surprise him, Ennis is worried that someone driving by actually saw them just standing there talking. There will be no hanging out in town even just for drinks because that's the way Ennis wants it.
I agree that fear drives him to it, but it's still selfishness.