>What is detransitioning?
Exactly what it says on the tin. Either MtF returning to being a cis man, or FtM returning to being a cis woman, reversing the effect put into transition.
>Why do people detransition?
It's more complicated then simply regretting transition. It could be lack of familial support, introduction of hardships due to said lack of support. Sometimes, trans people look at the life of their birth gender with rose-colored glasses, realizing they never got to experience it fully.
I have a problem... I'm a cute passing trans girl, I frequently get laid with both men *and women*. I've stuck my penis into vaginas. But, recently I've realized that I may actually be more attracted to women than men, and this completely disrupts everything. I realize that I never got to experience being a man... I went from being a pathetic little boy, to a woman. I want to be a handsome man who can make girls blush. Since I have experience living as a girl, I would be an even better man since I understand women better. I want to get a well-paying job, find a beautiful woman I love and take her in, have sex with her vagina every week and marry and impregnate her one day.
Pre-transition I was a pathetic manlet with no friends, who couldn't get laid. This is probably a big part of why I transitioned, but now I have the social experience from transition so that I could give being a man a 2nd try.
But what if it turns out I really am trans? What if I come off the hormones and realize being a man makes me depressed, or I turn out an ugly guy? Then testosterone already ravaged me and if I re-transition I'll be an ugly woman and probably commit suicide after this mess. Not to mention that I started hormones at 17 so I'm significantly behind the average male in physical development, it would take years before I could enjoy my fantasy of being a handsome man who makes girls blush.