I don't know if I'm unique in this regard, but I actually find it rather fun / interesting to work on the "rewrite the legacy system" projects. I generally find it to be an interesting puzzle.
It takes a lot of patience but it is oh so rewarding. I like to think of it as "software archaeology".
It's fun to gradually discover the model that the programmer was intending to program, then to see where it broke down X months/years later and was fixed with a hack. Then, you get to design a new model that fits both cases elegantly, instead of just a subset.
You kind of feel like the scientists in Jurassic Park! :)
My favorite activity (in life really) is to "make order out of chaos". Refactoring from tiny feature to full rewrite is prime example of this. Adding tests, adding documentation, all of it. Loves it!
In real life I get "chaos -> order" highs from organizing my collections, tidying up house, doing dishes, etc.
If I look around code base or my house and there is chaos I get stress. If everything is "in it's spot", ordered, understood I literally feel wave of "ease" wash over me, I am able to relax.
I like working on legacy systems too, but not to rewrite them wholesale, as that just tends to cause more problems --- instead, to carefully maintain and extend them.
