One more cause: addiction.
Food is associated with family, love and generally good fuzzy feelings. Add to that the addictive qualities of sugar (and we have some serious sugar overload in our foods) and the general principle (at least here from the generation after the war) to always clear your plate and well..its a recipe for disaster. Food can act like alcohol, drugs and all the other addictive substances, in that it is truly easier to take a trip down memory lane stuffing your face to experience that feeling of contentness, love and warmth, than it is to confront whatever you are running from and actually work through it. And really..it is a lot easier to stuff something in your mouth and literally eat the emotions you are feeling, when someone is screaming at you that you are a worthless **** and you need to get your act together, and make yourself temporarily functional again (since you dont have to deal with the emotions anymore), than it is to tell them to fuck off, give you some space and let you work out what the hell is going on. Especially if you were raised that how you *feel* isn't exactly relevant to *anything* *ever* and you need to get with the program NOW.
On top of that...you have to eat 3 times a day and binging is triggered when you do not eat, which screws up your metabolism.
Iow: it is a vicious cycle. You eat because you feel lousy, you feel guilty coz you overate, which makes you in turn feel lousy and eat again, your metabolism gets all screwed up, your body asks for real food (and not the sugary fatty kind) making you feel lousy again, which makes you binge again. And your life becomes controlled by food. Both deprivation and actual physical pain from overeating. Deprivation becomes your worst fear, as food becomes your best friend/ abusive husband. Sure, it has horrid consequences (but in your mind you deserve them anyways) but at least you will be able to deal with whatever come your way, or so it feels.
It becomes a habit, as it is a pain you can control (and use to punish yourself, by deliberately overeating when youve disappointed yourself). Which is preferable to the emotional pain that you have no clue how to deal with since you were never taught properly. In this, it is akin to cutting yourself.
Add to that the fact that fat people are often treated worse than lepers and seen as lazy and dumb by society, as well as the fact that it aint like you can quit eating (like you can quit drinking for instance in order to fix the problem), as well as the fact that your guilty pleasure is broadcasted for all to see (after all, you carry it around on your body, its not like you can hide it, like a gambling-problem for instance) and the guilt, shame and misery your psyche gets submerged in only pushes you down faster. It is like wearing a shame-billboard, to invite bullying.
And so you eat more. The heavier you get, the less you are inclined to get out of bed (digesting is actually energy consuming like mad and so your body feels sleepy all the time), or do something, contributing to the laziness-stigma, making matters worse, as well as your libido dropping, both due to the apathy and the body-image issues that follow and your interest in exercise goes out the window with all of that as well, since you cannot muster the energy and your body is so out of shape that it seriously becomes a frigging mountain you have to climb every time just to get started. You effectively become a prisoner within your own body.
The worst part is that, like any addiction, you have to want to make the change and somehow find the strength and conviction that a) you *are* worth fighting for, b) you do deserve better and c) people are idiots so who cares what they think. As most of these people usually suffer from incredible low self-esteem, and incredible sensitivity to guilt-tripping and shaming, it can be immensely hard to crawl out of this pit.
For loved ones, the pain of seeing the person they care for suffer and inflict that suffering upon themselves can turn their love into resentment and despair as they find all their good intentions going down the drain (since those are often experienced more like more guilt-tripping and shaming, with the person finding themselves unable to live up to their expectations, in essence only making things worse). There *is* no way to help them, except for support them in their every move, and not getting disheartened at their stumbling and relapsing along the way, which most people are not cut out for.
****
Before anyone asks, this disease is rampant in my family, hence my knowledge on the subject. My aunt, for instance, is at the point where she can no longer take care of herself and has been stuck in a depression for over 15 years, at the very least. And she gains weight *every* year, is unable to keep herself clean, clean her house to the point where she actually smells. It has destroyed her life. And she isn't the only one, unfortunately.