Because being overweight can cause certain diseases like heart diseas, diabetes, cancer, high blood ressure. These all reuce the quality if life and reduce life expectancey. So to be an ideal weight for your age and build means you will live longer and enjoy it too!
Because being obese puts a huge strain on your body and puts you at a high risk of a number of diseases such as heart disease, diabetes and hypertension (high blood pressure). You can also develop breathing problems while you are sleeping. Your heart has to work twice as hard when you are severely overweight just to get you from A to B.