In a word, guilt.
Back in high school one of my associates I played tabletop games with was big into the show (and, I have learned recently, was only pretending to be into the show for a girl he liked, talk about bad reasons), and while we were playing these games with others, and would usually fall asleep, even while involved in a critical role. Well, I took it on myself to try to fix this so the group, and me, would be able to have him actively contribute to the party and stay awake. So, I did some light research into the show and wasn't that impressed with the walls of text.
Next time he was falling asleep, I brought it up and made some light fun of it. It didn't work however, as he fell asleep anyway and everyone was looking at me like I was crazy. Regretting wasting my, and everyone else's, time I let him and put the incident out of my mind.
After I had finished high school and college was drawing near, I was reflecting on what I had done during high school and that incident came to mind. I decided to pull up the show, might as well see what he found interesting, or, at least justify myself my previous behavior, and promised to myself to watch the entire first episode. Well, the first episode wasn't anything to write home about, though it did provide interesting lore. I moved onto the second half of the episode, and, while it was better now that the exposition was out of the way, still wasn't anything to go crazy over. At least until Her Majesty walked into the picture, and forgave Luna of her actions, without serious punishment. The part where Celestia hugged Luna sealed the deal for me. (The story with Celestia is something worth a post all by it'self, but that isn't relevant to the thread)