People still need to keep themselves entertained which I believe is the main purpose of a job. We even see it now, people that retire can become quite depressed and quickly try to find some kind of partial job where they can feel useful.

So in a post scarcity civilization the main reason people will still work is so that they do not blow their brains out. The difference this time is that we will work on what we like, or on our calling. Now that I think about it, jobs themselves may be the scarce thing. i.e. Not everybody will be able to work on designing the next space station and hence there will be fierce competition to get these jobs.