Children cause their parents grief, trans or not. If a friend picks an issue with their child, they will defend them categorically. At the end of the day, you are family, and that person is not.
Same goes for siblings. Even when they get along really well, there are still issues. I remember my cousin getting a tizzy over what someone said about his sister, despite there being an 8 year age difference and he had always told everyone he wasn't fond of her. To this day they are more distant than close. But he still got all worked up because she was family. And it's what family does.
I realise you transitioned very young so I'll explain this as best I can as someone who is barely starting transition at 30. One of the big developed masculine traits that not a lot of people talk about is the need or desire to 'protect' someone. And not just from physical violence, but also in regards to 'honour'. I myself developed this trait during my 20's and am particularly fervorous when it comes to my Mum and Grandmother. If anything would happen to them, such as someone shoving past them in a store, I'd just drop my normal timidness and fear and shout at said person or go up and stop them if it was particularly serious. And it's completely against my personality otherwise.
You may want to look at it like they enjoy looking out for you. Like one would do a pet. Sure, pets take a lot of effort and money to look after, but after a day of taking for walks, scooping poop, getting those shots at the vets, digging up Mrs. Jones' petunias, feeding and then barfing straight back up - and you're just laid on the sofa cuddled up together, does any of the bad stuff really matter?
Nope, not in the slightest.