Seeking to broaden my horizons

So I am a 25 yo, cis straight male. I am not questioning my gender or sexuality. I'm a pretty open minded and tolerant person, but for whatever reason I find it difficult to empathize with the trans mentality. I think this stems from a lack of understanding. As someone who understands mental illness, it seems like a very trying situation for trans individuals and their families.

I am familiar with how the DSM delineates Gender Dysphoria, but I guess my question applies more to the underpinnings of why individuals feel uncomfortable in their biological gender. Is the desire to embody the opposite gender based on a societal or sexual premise? Is it rooted in the desire to live to fulfill the gendered norms and tropes of the opposite sex? Is it sexually based, in which the individual cannot feel sexually attractive/appreciated in the birth gender? Is it something else? Both? Neither? I feel like I'm missing something.

I know the answer to this question will be different based on each respondent, but I am still curious as to the range of different experiences.

P.S. I sincerely apologize if I did not use the proper terminology in my post. If I did, it came from a place of ignorance. I mean no disrespect. I took a gender and women studies class once, but beyond that this is all pretty new to me. If I did make any mistakes, please correct me.