What UBI assumes is that:
1) All members of society should share in productivity gains.
2) Working for someone else isn't the only way of being productive.
3) Money isn't the only --- or the best --- way of attaining status or self-worth.
4) Most humans have a desire to be productive in some way, and that desire can best be fulfilled in a self-directed manner.
5) There's plenty of fulfilling work available, even if that's just participating in vibrant relationships and communities and taking care of our homes and our hobbies; we don't have to make work as if we were in 2nd grade and the teacher needed a break so he or she gives us those busy-work assignments most of us hated.