Tires.
I used to never come close to wearing one pair out before wanting to try something new and different.
But now I have pretty well settled on RH Barlow Pass for my road/gravel bike and will soon be buying my 3rd set.
On my MTB, running 26” means there are few new and shiny things to tempt me, and I’ve pretty well settled on DHF for the front, though I’m still in search of the unicorn rear. I think the biggest thing I may like about my next MTB will be all the new options in 27.5 and 29.