It is hard for some people. Weight gain can be genetic. For instance, I'm a teenager who has always eaten fairly well. I still gain weight if I eat a 1,000 calorie meal without exercise. Most girls my age don't have to worry about that. Also, some people's metabolisms adjust really differently.
Some people also find it hard to find time to exercise. They also don't know what foods really are good for them since the companies that make unhealthy foods are constantly trying to promote them as healthy in some way or another.
Weight loss isn't easy. No lifestyle change is.
My favorite healthy food is peanut butter.