Because yesterday was a bit of a slip up, and I need to get back on track today. I need to keep going. Losing weight makes me feel good, so it is worth the "sacrifice." It's worth the extra work.
I'm staying on plan tomorrow (as it is 10 at night and I shouldn't be eating any more) because I went off plan at breakfast (well, not technically, but it wasn't the most healthful, satisfying breakfast) and I was cranky and felt like crap the rest of the day, and really had to fight to stay on something even moderately resembling plan.