Why do some kinds of work not feel like work?

A topic of­ten dis­cussed here is how to avoid akra­sia/​pro­cras­ti­na­tion in or­der to get on with work. I sug­gest an­other pos­si­ble “workaround” for akra­sia is to find work that doesn’t feel like work. From per­sonal ex­pe­rience, I know this is pos­si­ble, be­cause many of my efforts did not feel like work, in the sense that my mo­ti­va­tion on those pro­jects was so high that pro­cras­ti­na­tion sim­ply wasn’t a fac­tor at all. (I re­mem­ber, for ex­am­ple, de­sign­ing parts of my open-source cryp­tog­ra­phy library ev­ery day while walk­ing to and from class, and then cod­ing as soon as I got back to my apart­ment, or later, think­ing about mul­ti­verses and an­thropic rea­son­ing in much of my spare time.)

Why do some kinds of work feel like work, while oth­ers don’t? (Is there any ex­ist­ing liter­a­ture on this topic? I tried some searches, but don’t re­ally know what key­words to use, so I’ll just gen­er­al­ize a bit from my own ex­pe­rience, and open the ques­tion for dis­cus­sion.) Among the pro­jects that I’ve done, the ones that didn’t feel like work seem to have the fol­low­ing in com­mon:

  1. It was in a field that I found in­ter­est­ing and ex­cit­ing. (What de­ter­mines this seems to be an­other in­ter­est­ing mys­tery.)

  2. There was no pay­ment or other form of obli­ga­tion to com­plete it.

  3. There were no nega­tive con­se­quences for failure, other than time spent.

  4. It fit my ideal­ized self-image (e.g., cypher­punk or am­a­teur philoso­pher).

  5. There was an im­plicit prospect of sta­tus re­ward if suc­cess­ful.

  6. I hadn’t done it for so long that I started to get bored.

Un­for­tu­nately I don’t have enough data to con­clude which of these fac­tors were nec­es­sary or suffi­cient, or their rel­a­tive weights in con­tribut­ing to the “not work-like” feel­ing. Do oth­ers have similar, or per­haps differ­ent, ex­pe­riences?