In my case it's that I can't take the first step. Once I do that it all snowballs.

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Here's a light-hearted but sus specific example of what shartcracker wrote about from my experience:

Modded Minecraft, FTB Packs like Infinity Evolved or Revelation; when I get to the point where I have to put in real effort to build storage networks I always stop and come back like 1-2 weeks later. It's not even that hard to do thanks to mods like Refined Storage but the first couple stages are still annoying, followed by hooking up all the machinery and farms, and making sure I use the right upgrades etc..

Once all of that is done, however, I stop playing altogether for like 6 months because I've overcome the biggest challenge of a modded-MC playthrough for me, lol.