Just want to say that not all fat people are lazy. I know that weight gain is all about the relationship between energy in and energy out, but some people (me being a good example) have multiple health conditions that mean losing weight is very difficult.
Amongst many other issues, I have pituitary issues leading to:
Hypothyroidism, causing
Tiredness
Weight gain
Constipation
Aches
Feeling cold
Dry skin
Lifeless hair
Fluid retention
Mental slowing
Depression
Hypogonadism, causing
Decreased muscle mass and strength
Osteoporosis
Fatigue
Difficulty concentrating
Growth Hormone deficiency, causing
Low energy
Decreased muscle mass and strength
Weight gain, especially around the waist
Feelings of anxiety or depression
Thin and dry skin
Hyperprolactinaemia, causing
Bone loss
Decreased muscle mass
All the symptoms marked in red contribute either to weight gain or difficulty losing weight.