Keeping your weight in check
Being obese -- considered a BMI of 30 or more -- generally (though not always) means putting yourself at risk for life-threatening diseases such as diabetes, heart disease, and even cancer. Morbid obesity is where things get really risky. Come on, they put "morbid" in there for a reason.
The good news with this one is that you can get some extra time on the clock. If you're overweight and lose some pounds, that could actually add years on to your life.
On the flip side, overweight (a BMI of 25-29) people have been proven to have a longer mortality compared to average-weight or obese people. So although it's important to maintain a healthy weight, if your body naturally holds on to a few extra pounds, that could actually be a good thing. To sum it up: obesity = bad, morbid obesity = really bad, normal weight = good, and being slightly overweight = also good. Science, man.