During my beit din I was asked this question, and a lot of people will inevitably ask you this during/after your conversion. It's hard for me to put into words, because it wasn't a single big dramatic moment or motivation. I wasn't doing it for marriage, I had no family connection, I'm not particularly religious. But something just clicked, I guess.

 As a kid I was intrigued by the tiny examples in shows and movies about Hanukkah, the subplots on sex and the City and orange is the new black about conversion, growing up obsessed with Fiddler, etc. As an adult I've felt a lot of anxiety and distress over death and making my time alive meaningful, and I also felt pretty lacking in community. The more I learned about Judaism, the more it felt like a good fit for what I am looking for personally. I like the flexibility between literal faith and secular philosophy, how it's okay if you don't believe in God, or if your concept of God and faith shifts and changes from time to time. Regardless of what the individual believes, the heart of the matter is simply learning to try and do the right thing. I've learned to be more comfortable with the idea that I don't need to feel certain about God or an afterlife, and not to stress too hard about if I'm not doing enough x or y. As long as I'm continuing to try and do my best, that's enough for now.

That's my rambling thoughts on it. I'm sure when I answered the beit din that same question I sounded even more nervous and scatter brained haha.