For me, it was seeing everybody else enjoying life when all I could do was dream of being somebody completely different than myself. I'm not the type of obese person who just became obese through time. I literally was always obese. I never experience anything a normal girl experience. I have an imaginary 'perfect' version of myself that I always dream about. I'm so sick of dreaming... I want to live!
And than seeing my brother (who I really don't like due to something completely off-topic) losing 40lbs and being thinner than I am and my cousin (who I also don't like) being thin himself... I just couldn't take it anymore.
I also realized that I have type 2 diabetes. I've been diabetic for a loooonng time... Probably before I was in 2nd grade (I was obese then too). I just didn't realize it because I was too busy dreaming! Because of my obesity, I have a lot of ugly side effects. One of them is acanthosis nigrican.