To begin with, the mullahs have an expansionist authoritarian religious ideology which is at odds with almost the entire world. Their only real allies are Syria (a Shia minority-ruled dictatorship) and Hezbollah (Shia terrorists). Every other "ally" is only an ally because of shared enemies (Venezuela, for example). Western liberal democracy is viewed as intrinsically threatening to them because it seduces Iranian youths who want to emulate permissive western behavior.
But more immediately, Iran has an interest in acquiring nuclear weapons, they are hostile to our ally Israel (a hostility entirely initiated by themselves), and they have an interest in encouraging regional instability. Such instability helps prop up oil prices, which they want as high as possible, while we want oil prices as low as possible.