That was an interesting read, but the title question is never answered or even addressed substantially. Articles like this sorta piss me off to be honest. I'm a combat vet, and the subject in my mind just by reading the title became flooded with questions about what kind of things would be said in the article... and then I read it. It good for what it is, a story about a transman's journey of fighting in a boxing ring, but the interspersed poetic questions about why men fight are'nt answered, and are just left hanging, though I can tell they are intended more as conversational glue.
Men fight because since time immemorial violence was (and, to an extent, remains) extremely effective for securing mates.
Although male aggression is not tied solely to it, testosterone injections should help a sufficiently reflective transsexual understand this over time.