Pretty regularly, but not in a negative, fixation kind of way.
When my mind wanders, one thing it tends to drift towards is "how will I respond if ____ dies early?". It could be my wife, daughter, brother, friends, parents, other relatives, whatever. I sort of run myself through what things I would need to do or take on (particularly in regards to my wife and my parents), so if that day comes, I'm not just flailing.
It sounds kinda morbid, but I hate being caught flat footed by a surprise event, and there really isn't anything that can do this quite the same way death does.
I also have a lot of experience with untimely death. 3 of my first cousins (same side of the family) died before they were 17. 2 of them were brothers who were never alive at the same time. I was very close to both as they lived just down the road from me and our dads (brothers) farmed together, so we saw each other almost daily. The third one had a rare brain cancer. All of these occurred before I was 22, and had a really powerful formative influence on my life. You just end up looking at death a little differently after experiencing stuff like that.