He fell in love with her during that very first visit to the park and I think it was suggested that the park gave him a feeling that the outside world never did, seems he was really unhappy in his life. Her being wiped and forgetting him made him become jaded and cynical (and these negative traits increased and increased over the years) and he kept visiting the park due to how much he preferred it to his real life and to find a way to make it more real (so he was genuinely happy when Clementine shot him at the end because he has his wish now)
Oh I thought your Arnold theory meant you'd figured out the timeline craftiness because we were told Arnold founded the park with Ford years ago But anyway I'm glad you didn't pick up on it, that was one of my favourite twists
Since you thought William was Arnold (think that's the first time I've heard that one ) I'm assuming you picked up on his story taking place years earlier before the reveal?
Yeah I've never been clear on that myself. My personal thought has been that the phrase was programmed to trigger her "escape" storyline, like it triggered Dolores to kill Arnold