As they say, keep your friends close. I've enjoyed Arthas/Lich King lore since Warcraft 3 and wanted to experience more. I levelled a Rogue straight to level 55 and then logged off, created my DK and haven't looked back.
I still maintain a close friendship the the Lich King. I never really wanted to join the Horde, and my darkest day was having to kill him.
Now I just bide my time before being called upon once more.
Used to be a mage back in vanilla, didnt like how squishy they were and plus i was bad lol. DK's had come out when I got back into wow tried them and I just fell in love with the play style of them and since then have been a DK since I came back.
I was a ret pally from the end of vanilla until they introduced holy power and ruined the class, switched to dk and never looked back.
Lore, good looks, power.
I always wanted a plate warlock who used melee weapons. And they gave it to me.
I'd prefer my orcs to stand up straight.
Honestly, I originally only made my DK to solo old raids, since I mained a holy paladin, I could do them it just took ages since I didn't have a dps/tank set and didn't want one on my pally.
Later down the road though I turned it into my raiding main because I loved the feel of Blood...
I basically changed "mains" each expansion. Vanilla was Shaman, TBC SPriest, WotLK was DK. Basically stayed with those since. Though I'm leaning Mage for WoD.
My DK never had a fair shake as my main, as I was always the guy who could be relied on to heal, but Unholy... was always just freaking awesome. Everything about it appealed to me.
If I had the realistic option to swap to it I probably would and no matter how bad Unholy dps is or how much easier Frost is... Good ol'e Timmy and I would be rocking it.