I used to dislike fats, mostly because of their lack of discipline. My train of thought was, the only reason they are fat is because they eat too much, and so long as they're very careful with their calories and stay very active, the issue is fixed.
Working out has changed the way I thought of that. I now have first hand experience what it takes to gain weight. Let me tell you, it's not about eating a bit too much, or having that extra late-night snack, or about drinking coke instead of diet coke.
You need to systematically overeat, morning till evening, every single day, to gain large amounts of weight. You need to stuff your fucking face to forcibly change the amount of food your body is used to. That's what I learned.
You don't become fat "by accident", but by long-term, systematic, severe fuckup. There is no excuse.