I'm going to try not to sound corny here, so bear with me.
I RP for a few reasons. One being that I love telling stories. I love reading fiction books with different worlds, whether it be fantasy, sci-fi, horror, or mystery. I've often read a book or graphic novel and thought "Wow, being part of this would be cool!" But of course, I can't live like a superhero.
But with RP, I can! Or rather, I can live through my character. Each character I create has a little part of 'me' in them. It could be their look, their personality, their likes, etc. So I tend to live through my characters. I always try something new.
Second, I RP because it gets me connected with other people who share my love of interactive storytelling. I've met many people in these communities who have been so much fun to work with and interact with. The stores they create are sometimes awe-inspiring.
Lastly, I RP because I want to have fun. It's why I do anything really. It's why I play certain video games, its why I watch certain movies/tv shows. It's why I join certain games and ignore others. If I am not having fun, there is no point in doing it. It's like playing League of Legends or something. While it can be a fun game, in my experience, it's often led me to not enjoy myself because I end up playing with people who get pissed off if I miss one of my jobs or if I don't do something I should do. It's like....enjoy the game!
I've had a good amount of bad experiences in RP. I originally started RP in MMOs, with City of Heroes and I branched into other games. In most, if not all, of these MMOs, the RP community ended up being really hostile and volatile with how they treat other people. So I gave them up and decided to try something else because, in the end, I missed RPing.
It's been a tough journey, but I enjoy what I do now. It continues to be a fun hobby.