Because that's how his/her brain works.

Speaking for myself, the negative things about female pale into insignificance compared to a disjointed mind and body, which i would describe as a form torture, that i wouldn't inflict on my worst enemy.

As for not behaving particularly female, that's a pretty standard thing, as a lot of us suppress(ed) those feelings and try to behave more like our genetic gender, because that is what society expects.

My advice would be for him/her to talk to other trans people, then what he wants might become clearer.

I would recommend something similar for yourself, there are groups out there that cater for partners of trans people, and they might be of help for you as they will know what you are going through.
Take care