Why is it easier to keep from eating M&M's "on your own" and leverage that ability to motivate you to do work, than it is to keep doing work "on your own" in the first place?

I'm assuming because "don't eat undeserved M&Ms" is a clear, simple and binary rule - breaches are obvious, so there's not much of a temptation to rationalize them. "Work on my stuff" is broad and fuzzy and has plenty of room for excuses like "I'm a bit tired today", "I deserve to rest", "I'll do it tomorrow', etc.