Challenge, anyone?

Lucas and I have started a 21 day junk food free challenge and I have to say I think it was just what I needed. I have restarted my food tracking in MyFitness Pal and so far so good. I am actually enjoying it. Why is it that I always forget how much better I feel when I eat better and that I actually enjoy eating well? What is it about eating junk food that triggers my brain to make me crave that lifestyle? I just don't get it!