Your body is NOT meant to carry around hundreds of pounds. Your blood circulation becomes compromised, causing higher blood pressure (your heart has to work harder to pump the blood, so heart failure) and potential blood clots.
Diabetes, bone degradation, lymphedema, skin infections, breathing problems, sleep apnea, and at least 4 types of cancer have been linked with morbid obesity.
Obesity and smoking are usually neck and neck for the #1 spot of diseases/death caused by preventable circumstances. This means people die from something they could have avoided or quit early on to prevent disease, cancer, and death. I'm not saying that EVERYONE who is obese will die or that EVERYONE who smokes will get lung cancer, but hell, you're certainly digging an early grave by keeping it up.