As someone with that issue, it's usually our upbringing. Parents who don't give a shit, or actively discourage the betterment of our lives. It also happens when some people have the higher metabolism and don't gain a fucking pound no matter how much carbs and sugar they have in a day, every day, think that their offspring will be the same and then "Whoa, why the fuck are you so fat kid? You're a lazy fuck, I'll bet." when they haven't taught their kids about any of that stuff and eat like a heifer themselves while lazing about all day. I have tried so many times to change my eating habits, but my family makes it damn near impossible between the fact that I'm the only one in the house to cook food period, the fact that my remaining grandmother has diabetes and I have to basically force her to eat even moderately well when she's super fucking picky and swears up and down we're abusing her if we don't fix her what she wants every night, the womenfolk screeching that I'm starving myself if I don't eat tons of carbs a day and having to listen to the women scream about not having sweets on hand (Though, mostly my Grandma). It's highly stressful, and I don't have it in me to cook something that I would prefer for what I would like for my Dietary plan after dancing like a monkey for the womenfolk in my house hold. I usually don't have the money either to buy my own set of groceries when everything is said and done (Bills, gas, general groceries and necessities, etc.)