This was my long, rambling response to a forum post , before I decided to edit that down to provide an actual answer to the question being discussed. However, I thought I’d stick this up here since it’s a nice summary of my experience with Merlin. I would be happy to hear from any readers about how you discovered/became obsessed with the show as well (or why it never became an obsession for you...maybe you have some pointers I could benefit from!)
I only just discovered Merlin in January of this year and as a result, experienced the characters' 10-year journey in a matter of months. There's something very powerful about that, I think. My obsession was a bit of a slow burn, however, as I was only mildly interested in the show at first, didn't really care for the main character, and didn't find the actors all that attractive. It first piqued my interest with "The Moment of Truth", where the friendship between the four main characters took front and center. It might have something to do with the fact that that episode also majorly teased the magic reveal. The showrunners may not have been right to make that their endgame, but they certainly were on to something when they considered it a huge driving factor of interest in the show. I hadn't realized how tantalizing the idea of a reveal was until then, but from then on I always noticed my breath catch in my throat whenever a moment came where I was sure Merlin would be caught in action.
Then there were the little niggling details that got me curious. Why did the costume department decide that Merlin should wear a bracelet? Did the makeup department have something to do with making his ears stick out that much? Were the girls' dresses period-accurate? This led me to behind-the-scenes videos on YouTube, and again, I was impressed by the camaraderie among the core 4—though this time, the actors instead of the characters. They just made their jobs look so fun, and it reminded me of my own childhood dreams of acting. I think the success of the actors and the friendship among them (or just the characters, for those who don't also obsess over behind-the-scenes stuff) is something that many fans admire and aspire to. It hooks into your heartstrings and you want to see more, and having that taken away when the show ends leaves you feeling like you've lost your friends as well.
As I was into seasons 2 and 3 by this point I was truly enjoying the show's excellent humor (trolls and goblins and Gwaine, oh my!) and overall well-written and amazingly-acted episodes like "The Fires of Idirsholas" and "The Last Dragonlord." As I was increasingly stressed with work in my real life, the show was giving me a much-needed escape. By the time seasons 4 and 5 trudged along, I was hooked, and I was more than willing to fill in missing details or rewrite scenes in my head that I thought should have gone differently. (I even suddenly and for no reason, found the actors more attractive. I am still baffled by this change of opinion. Must be sorcery!) When the writing was good, it helped me escape, and when it was bad, it inspired my own creative mind. And when it was particularly tragic ("The Disir" or "Oh crap did they actually just decide that Arthur's death was Merlin's fault?!?"), it inspired me to search out online communities to find others' contradicting opinions to alleviate the way that my interpretation was ripping my heart out.
By the time I got to the finale, I’d had the fact that Arthur dies spoiled for me, and I was dreading it. But, it was a lot better handled than I expected. I was actually more or less pleased with the finale. We didn’t get the ‘golden age of Camelot’ with magic legalized that we’d been promised (of course, as I saw pointed out elsewhere online, Kilgarrah did say from the start that if Mordred lived, Merlin would not fulfill his destiny), but I felt that Camelot was certainly in better hands now than it was with Arthur. Especially since Gwen figured out Merlin’s magic in one day, when Arthur had completely overlooked it for 10 years! I was also mollified by the way they handled Arthur’s gradual acceptance of Merlin’s magic and the fact that the two spent one last episode together (as opposed to the ‘I’m dying’ – ‘I have magic’ – ‘I know’ – *dies* that I’d been expecting). ‘I must have overlooked Merlin in the final scene with Gwen,’ I thought. I watched it again the next day and still didn’t see Merlin, but I was still ok with how everything turned out. It wasn’t until I woke up the next morning thinking ‘Oh my God, Merlin wasn’t in that scene because they meant to say that he never returned to Camelot,’ that the finale completely ripped my heart out. Now I’m left lurking round online communities looking for alternate explanations to make me feel better, and just generally enjoying the opportunity to discuss and appreciate the show.
Side note: I’m now re-watching the series, and I’m intrigued that while the first time around, it was clearly about the platonic love story of Merlin and Arthur, this time I’m focusing more on Morgana’s tragedy and the paternal love story of father and daughter. Since Morgana's turn to evil was handled with much less care, I may have a different opinion of the show by the time I've finished this watch-through!