If someone is being a dick and doesn’t realize why they’re being a dick

I think you’re confusing the words how and why. Why they’re being a dick is a question for a therapist, unresolved childhood trauma, insecurities etc.

How they’re being a dick is the question you seem to be referring to. They don’t realize that what they’re doing is wrong or hurtful, because they don’t see how their actions are affecting their partner.