I'm similar. right now I'm "off" dwarf fortress. I haven't played in about 3 weeks and likely won't for a month of two. However, the entire time I'm away from DF, I still think about it, check the dev log and forums and mull any new things I can do in my next fort.
But when I'm "on," DF is all consuming. I will play for 4 months at a time, every day for as long as I can manage. I will run established forts in the background at work and play it like an idler game and only intervene when I'm needed. Then I come home and play for three hours making strides to improve the fort. I will think all day about how to improve my fort, how to do something new, build a new room type, squeeze extra productivity out of an underused industry (for me that's the clay industry), how to streamline my major industries.
And then that fort will die, I'll adventure for a little bit, start a new fortress and have this empty feeling. I think the empty feeling is two-fold, first, I feel empty because I miss my old fortress. even though I'll implement new ideas in my new fortress, I still miss the old dwarves. My forts tend to last 20+ years and I come to know and love certain dwarves. I will never forget Nish Uristinol or Sarvesh Cugnacen or Tun Olthilistat. When they died I was truly saddened. I ran another 20 year fort after I lost those friends but I just felt like I was going through the motions. after that fort died, I started a new one and haven’t gone for more than 2 years on that fort. I think I just need some time to mourn.
Second, I think the empty feeling is partially attributable to the world not progressing after world gen. I like to play in the same world for as long as possible and get a feel for its history and culture. However, my current world has been out of world gen for 50+ years. a lot of the historical figures are dying off and I've killed most of the things to kill as an adventurer. the world itself is dying and there's just not as much fun to be had because of that.
Ultimately, I think you've hit the nail on the head, it's just this urge. I've done this a few times and always come back.