From Bryan Caplan

Leftists are anti-market. On an emotional level, they’re critical of market outcomes. No matter how good market outcomes are, they can’t bear to say, “Markets have done a great job, who could ask for more?”

Rightists are anti-leftist. On an emotional level, they’re critical of leftists. No matter how much they agree with leftists on an issue, they can’t bear to say, “The left is totally right, it would be churlish to criticize them.”

Caplan explicitly calls it a simplistic theory, and it is. But it’s not bad as a first approximation — as George Hawley suggests:

If out-group hostility is more important to party identification than support for particular policies or ideologies, we may not actually place very many ideological demands on our parties. Defeating our enemies may be more important than advancing specific liberal or conservative agendas. According to Groenendyk: “If partisans’ identities are increasingly anchored to hatred of the outparty than affection for their inparty, electoral dynamics are likely much more fluid than many accounts suggest. Thus, insurgent candidates with questionable ideological credentials (e.g., Donald Trump) may be more appealing than one might expect in the age of ideologically sorted parties.”