I actually didn't like Halo at first. I went from "oh, that game" to it being my favorite fictional universe of all time... and it's completely the Sangheili's fault.
My dad played a lot of FPSs, and they really weren't my cup of tea, as the ones I was even allowed to play at a young age (my parents very closely monitored what I played up until I was 13-14) often resulted in me just killing myself with grenades or proximity mines.
When my dad got an Xbox, of course he was playing Halo, and ten-year-old me rolled my eyes every time I heard that darn theme play, and I retreated to my sanctuary of Pokemon, Mario, and Zelda on the N64 (Still enjoy these classics, btw, worry not.). He seemed to be having fun with it, though, so every once in a while, I poked my head out to see what he was up to. I recall sitting on the couch as he was making his way through Two Betrayals. Our conversation was more or less the following:
Me: Are you almost done with the game?
Dad: I think so.
Me: *sees a Banshee* What's that?
Dad: Basically a plane, and I need to really get away from it before I die.
Me: Oh, oka- *sees an Elite Zealot with a sword charging forward* Whoa! What's THAT!?
Dad: A very mean alien that WILL kill me if he gets close! Gotta go!
Me: ...He looks kinda cool.
A couple years later, I'm visiting a friend whose younger brother has one of the Halo novels, namely The Flood. A little curious after more incidents of watching Dad play, I peer through the book, and start reading, not knowing that it's more or less a novelization of the first game, but it clicks with me after a chapter or so. I finally learn the cool aliens I liked were called Elites, and manage to get to Zuka 'Zamamee's first scenes on the Truth and Reconciliation before it's time to go, and I dejectedly put the book back on the shelf, wanting to know more of the story. Sadly, these friends moved later, so getting a hold of the book would have to wait.
The next year, Halo 2 comes out, and my dad asks me, "Hey, this one has online multiplayer so we can play with other people! Wanna join as a guest?" I decide sure, it couldn't hurt. We go to make our characters, and my dad goes for a Steel-armored Spartan. Classic. I go to set myself up, and I notice the Elites are now playable models. Probably obvious what my choice was. We go into some matches, with me getting absolutely destroyed after I learned how to not walk into walls, but it was kinda fun.
A little while later, my dad's almost done with the campaign. I come out just as he's beginning Uprising, and we watch the intro cutscene. This was my first time seeing the Arbiter, and the questions wouldn't stop flying:
Me: Who is that? Why are you playing as him? Isn't he one of the bad guys? Why's he out there all alo-
Dad, with a hard stare: You know, why don't YOU play through the story and find out?
Me: ...Okay, I will.
From there, it was a hard and fast drop down the rabbit-hole. I now own all the novels (and some other merch), have read a handful of the comics (they're a bit trickier to find in comparison, especially the older ones), played through all the campaigns and am on multiplayer regularly, have autographs from a few of the voice actors, and still devour every scrap of lore I can get my hands on. Many of my closest friends around the world are people I would have never met had I not started playing Halo, including the person half across the world who would later become my fiance, and is now sleeping in the other room as I type this from our apartment, 12 years after we first met in an online lobby. While he also enjoys Halo, I think he smiles and nods as I happily fangirl over whatever crazy thing happens in the newest book.
And when I hear that theme song now... I'm usually humming along to the song that changed my life.