Personally I have been vegan for a year and a half and its mainly due to the fact that I recognise the wrongness in exploiting sentient beings by killing them so that we can eat them, etc etc. Also the DNA of an animal is so close to a human that it could also be considered a form of cannibalism if you eat them (think about that the next time you eat a mouth full of animal flesh)
I’m not fully vegan anymore but I also don’t like the idea of hurting and killing “sentient beings” unnecessarily. It also sounds pretty stupid to waste time, space and resources on animals to kill when we can just eat plants directly.
A known person has a blog post in spanish about that.
Is it possible for a vegan stop being vegan?
The thing is that most people confuse veganism to diet or animal welfarism when it is not. Veganism is such as considering individuals from other animal species also people, so not being vegan again is inherently impossible unless someone lost its mind (literally).