For me it's two things: illness and how I feel. I gain really quick with inflammation when I'm sick. And I'm tired and get lazy and unfocused about what I eat, too. The second reason is I don't find a huge advantage physically in being at my lower weights. I tend to get injured more frequently and I don't feel better overall. I think if I had physical motivation I could stay at that low weight. The last time I was that low it was because I was not eating due to a very stressful life event. I looked and felt horrible. My best physical conditioning has happened at 40-50 pounds over my low weight. But I still want to be thinner. It's a hard balance.