A tentative third for this line of reasoning - could this be 'need for variety' maybe? The more interesting my work coding is, the less desire I feel to hack on my own projects in the evenings - I'd rather spend the time juggling, spinning poi, or other activities that are quite different to what I spent the day doing.

However, coding is quite probably my favourite hobby - so if work is being dull then I start to need to do something interesting with code in the evenings to feel like I'm still indulging.