Why do people gain weight?
There's a lot of reasons, kinda a broad question. A few ---
-- Puberty, you gain weight and it goes away as you grow -- Over-eating/eating too many bad foods, it's okay to indulge once in a while, and normal to over-eat on occasion, but be aware of it! -- Genetics, this is also a reason. My dad was overweight, my mom was underweight. I think I have the fat genes, so I work hard to not gain weight quickly and keep my metabolism up. You can't completely blame this reason for being overweight. -- Not excerisizing, I excercise 30-60 minutes a day, because I'm trying to lose some weight. Most people should work out for 30 min/day, 3-4 days a week -- Diabetes, I'm not sure about this reason. You may want to look this up if you're wondering.
Please note: if you're very active, and you're gaining weight/overweight on the BMI scale, you probably have more muscle mass than fat mass. Muscle weighs more than fat, so talk to your doctor if you're concerned.
either becuase of lack of exercise, over weight kids, diabetes, also if its passed through the family... laziness, thoes types of things