I read an article about the obesity crisis I think by Caitlin moran where she talked about how comfort food is the emotional support many of us turn too. Cheap, easy to access, no hangover or illegal - food is there! If we are all a bit sadder, lonelier or poorer then some cake, chocolate etc is a quick boost.
I agree with this. It's the carer's disease. If you spend your life looking after others you can't afford a drug or alcohol addiction, but you can use food.
More generally I am partly in the "willpower" camp and I believe in personal responsibility. I also don't think the (very sad) traumatic experiences people have written about here are totally relevant when so much of the population is overweight. I was never obese but was overweight and lost 25kg over a year, and have kept it off. But I'd argue that willpower is a finite thing -
If you work a crappy job and expend all your energy doing it just to keep your head above water with little hope of your circumstances improving, that might reasonably sap your willpower.
If you live in a "food desert".
If you've spent the morning trying to make £30 stretch to a weekly shop and feel bad that you can't buy your kid x thing he wants for his birthday.
If you're exhausted and just want to get something on the table quickly that you know everyone will eat.
If you live somewhere unsafe/with limited access to green spaces let alone a pool or gym.
If you've had a shit day, with another shit day coming tomorrow and just want something nice/comforting/familiar.
If you feel unwell but keep getting fobbed off by your GP and aren't sure what to do next.
... etc etc.
I don't think any of these things on their own are dealbreakers, but I think a lot of people are using up quite a lot of their willpower just getting through the day, and there is plenty that the gov't could do to nudge people in a different direction. It'd be to the benefit of the NHS, though of course if you're heading in the direction of US-style healthcare then perhaps lots of ill people aren't such a bad thing eh?
FWIW I run a food bank and given the choice people do broadly make healthy choices when they come to us. We operate supermarket-style and people are delighted with bananas, apples etc. Last week we had both pineapples and asparagus in (surplus from a supermarket) - all went.