Complacency (or myself - take your pick). I think I've written about my fear of stagnation before. I'm in a fortunate position where if things never changed, that'd be okay. It's this line of thinking that terrifies me. It's something that I constantly need to remind myself to fight. Luckily, I think I've done a good job so far.
I've developed this pseudo spiritual belief that chaos (and so change) is the main driving force on life, without constant challenge we stagnate and die. You see this in human history with some of the greatest innovations born from necessity, the animal kingdom where evolution and nature favour those which adapt (or die), to the universe itself where no matter would exist if every atom remained stable. Change can be a scary word, it's naturally chaotic and we want to reject that as we think that must be a bad thing, but honestly as someone stagnating and gathering moss, not changing is scarier.