A side project is much different than a work project. You got your eight hours of work and got to go home and do whatever you pleased. In this case, it was to write more code. Having to corral your brain into the singular focus of an employer's project against deadlines, feedback (sometimes crazy talk,) updates, documentation and other distractions is different.

I can happily crank out code all day if it's something I'm doing for myself. As soon as you add the employer, it immediately starts doing my head in. It's completely different work. Some employers are better than others in this regard I suppose.