My daughter has been a fan of MLP since the G3 days. She's autistic and developmentally delayed so she was into G3 all the way to age 9 or so.
I watched the G3 show with her, usually patiently, but often not so. Then along came the transition to FiM, and at once I knew this was different. "Hey, this is a lot like The Powerpuff Girls" I thought. (little did I know at that time) And soon I was laughing my butt off, especially at "I got the tiiiiicket. I got the tiiicket." and "SHE KINDA IS!!!!"
But the moment truly came with Call of the Cutie, specifically the scene where Twist opens the door. Before that episode I still mostly watched the show because our daughter did. But seeing AB so totally destroyed with that little quivering lip made me gasp, "OH CRAP! What is she going to do now?!" That was the moment when I realized that I was truly hooked on the show because I had begun to genuinely care about the characters and what motivates them.