There are several reasons why. The first is that I feel a lot of shame about the way that I am. Whether that is right or wrong, I cannot say, but telling people feels like exposing the parts of myself I hate the most.
Secondly, although I am running this AMA, in my 'real life' I am a private person and like to have a quiet and mundane life. I have never felt that I was treated as a normal person by people who knew that I was trans.
I had some wonderful friends in sixth form college, and there was never any malice intended from any of them. However, it was very clear that I was seen as an oddity by my peers. It's exhausting to always be seen that way and I wanted desperately to just be seen as 'normal', after feeling so isolated and different throughout my childhood.
Finally, when I was younger, I went to work a job where all the staff lived on site. It was a move away from where I had been brought up and the first time that no one know about my past. I became close with a girl who worked there, and it was clear we both were attracted to one another. She attempted to kiss me at a work party, and I realised I had to tell her about my past, as it was unfair for her to be intimate with someone she didn't know to be female.
Whilst she promised not to tell others, it ended up being known by all and sundry. Because I lived on site there was no escape from it, and there wasn't anywhere I could go to escape the whispers and sniggers. There was one member of staff who took it much further than that, which resulted in events I'm not sure I will ever fully recover from. I have had an extreme fear of telling others since then.
I do think about how my friends would feel if they found out, and whether they would feel betrayed or would no longer want to be friends with me. It's something I worry about a lot and wrestle with to this day. I have never found an answer to the issue though.