Complexity keeps me there ...
BJJ is endlessly fascinating. Every week, we unearth a new development; perhaps a new technique we have never seen; or even more fascinating to me, a new 'nuance' or 'detail' that improves something we are already familiar with.
To again wheel out the old 'chess analogy' - even a little knowledge, when pitted against someone with no knowledge, allows us to prevail. But of course, the real fun is to be had against a knowledgable opponent - move vs countermove - the unfolding of strategy - gain through sacrifice - etc. Chess has a depth to it - a complexity that pulls people in for a lifetime; and so too does BJJ. Mathematically, BJJ has far more 'possibility' than chess - infinitely more ways a contest can unfold - and other factors coming into play like strength, fitness, etc.
BJJ is on one hand - beautifully simple - but the complexity is where the real 'gold' lies. That layer of 'invisible nuance' that we can 'mine' into and in doing so, come to learn not only more about how the art 'works' - but more importantly, we can learn more, much more, about how we ourselves work.