Im not that veteran player. I play the game for almost 2.5 years now.
I never actually wanted to start it, but a friend of mine literally dragged me into it. My first thought was : what the hell is this childish looking miniature running around hack and slash stuff ? Then i started playing and before i knew it i grinded snarbolax like a madman :D Childish turned into cute, and running around into doing missions or just having fun in arcade.
Gotta say, it got into my top 5 games of all times since then. And im still playing it. I took a 1-2 months hiatus once tho'. I cant even remember why.
So what keeps me playing?
For the most part, online friends from all over the world in this game. I still play from time to time with Tota-Batuta, who helped me through my first missions <3
Of course there are many friends, who faded away since, like Troyfel the brittish guy with his great television voice and accent who taught me the basics of FSC and speedrunning Vana.
I actually always found something new, something interesting in the game. I liked challenging myself into reducing time of missions, or even a full arcade run. But most challenges were of course about events, and Vana. Like for example do D27 last room with all 4 trojans dead statue still not destroyed... or doublekill the spawning trojans with 1 blitzcharge... etcetc.
Always cherished the moment, when i found a great player who i didnt know yet, and could play with from then on.
And of course (like my mentors helped me once) eventually i started to help others in missions be it casual play, or serious grinding. And most of the time i really enjoy helping people. Specially when it turns out that the person wants to learn more and more about the game. And after some time they end up wanting the same thing like me: speedrun FSC the faster the better... Yeah probably not the most crown-efficient way, but its not far from it if you play it good. And the main point is that its deffinately the most fun way.
And naturally even nowadays after all those runs... every now and then still comes a player and shows something new to me, something that i see and i want to learn from them or just thanking that they showed it to me. :D
Besides those things of course i see the games problems and difficulties. But i cant really help them, so why worry about them? Lets just do one more run, shall we? :)
So yeah its rather strange but i still enjoy playing this game. And probably for the reasons above.

I'm nuts.

Complex answer:
I had come to terms that there's nothing more to come, but I enjoy what's already in front of me.

You were quite polite in your original post, but this question is asked pretty frequently. While players give you their most recent answers in this thread, you might want to check out old threads on the same topic, for example by searching Google for what keeps you playing? site:forums.spiralknights.com.

Guild upkeep is a pretty big reason why I play the game on a weekly basis; it's a commitment thing.
Otherwise, aside from the obvious reasons of artwork and other such finery, I'm still playing after reaching the 2k hour mark because of the fact that my Spiral Knight avatar is something I got sentimentally attached to. The Warmaster, Iron Slug, and Gorgomega keep me interested in PvE, while the various darky accessories I have equipped are nice to look at and cause me not to feel too intimidated by the big-shots in Haven with more CE than I'll ever own.