To them, you have a made a conscious, avoidable "lifestyle choice", which causes them embarrassment and social ostracization. They don't understand that it's not a choice to have been born transgender, and they conflate your choice to live in congruence with your gender identity, as a choice to "become transgender".
They have no frame of reference, being cisgender, so explain your actions from their cisgender point of view.
The conclusion to them, being that you are making a selfish decision. Rather than question society's ignorance and transphobia, it's easier for them to blame you for going against common wisdom, that the transgender condition is a mental delusion.
It's a horrible situation to be in, but I try to understand it by remembering how I too, blamed myself for being transgender rather than question society's ignorance and transphobia.

Stay strong! You're not alone.