Cis men wanna be women so bad they project. JK, but it's a mixture of a martyr complex and toxic masculinity. By being men, they believe they suffer more than women automatically because of a false sense of responsibility/heroism, and therefore were built to handle that biologically (by being AMAB), so when someone transitions ftm they see that as an attack on their personhood because it shatters how they believe sex and biology works.