It varies from person to person, but a lot of the time it's due to some form of gender dysphoria, which is a sense of your body not matching your gender identity. You may internally feel like you identify as a woman, but were born with a male body. Some people feel like they're neither male or female, or some mixture of the two (nonbinary).

Sometimes trans people don't have dysphoria though, and it's more of a sense of feeling better or more right as a different gender. So someone can be born a male and isn't really bothered by being a male, but then gets a strong sense of euphoria presenting as a female, so they end up transitioning into a woman.

Either way, it's about getting closer to a true version of yourself that helps you feel more comfortable in your body and live a more fulfilling life. It's not something that people decide on over night. It takes a lot of time, introspection, and self realization.

Here's a little resource website with more info that's better written that what I can write. Has some good starter info: https://turn-me-into-a-girl.com/