I've noticed a disturbing pattern in my motivation to play with Lisp implementations. Now and then I fall prey to a new idea and start writing one, and it goes fast at first. I dash off the basic interpreter, perhaps an experimental representation of environments, a new macro system, modules, primitives... And then I get the the point where the kernel is basically working, and I need to write enough library - macros, mostly - to get some simple programs running.
And I stop. I like macrology. I like rebuilding the foundations of a language. I like seeing programs running on a tool I built myself. But as soon as I switch from writing the language to writing in the language, I lose interest.
This applies to implementations in an existing lisp, but even more to ones in C. I'll happily spend all day writing a garbage collector, even though I know it's an unoriginal waste of time. But when I start building library infrastructure, an understudied area where I could make a difference, I get bored.
Could it be that I actually enjoy low-level hacking? It is superficially rewarding, because it presents a constant supply of easy problems to solve, and they can be solved in familiar ways. It's programming candy - the fun of debugging without the pesky intellectual challenges. In C I can enjoy constant victories over my tools, whereas when growing a Lisp I'm confronted more directly with the problems. And they're sufficiently ill-defined and hard to think about that I recoil and go do something else instead.
I am not a good programmer when I prefer irrelevant problems to those I want to solve. I suspect this is a common tendency, and it may explain some of the puzzling lack of enthusiasm for more expressive languages. Who wants to face new challenges when you can keep solving the ones you know?