Several people on this thread have stated that type 2 is not always caused by lifestyle, because they or someone they know has it despite being a healthy weight, active, and eating a healthy diet. As well as the ones with drug or surgically induced T2.
Those posters are also implicitly saying that, when someone who is overweight/obese develops T2D, then it is their own fault. That’s what it boils down to, doesn’t it? And, by further implication, that people with T1, or T2 at a healthy weight, are more deserving of sympathy than obese people with T2D.
There is, as @Gutachenow points out, abundant research showing that obesity is complex, multi factoríal, and not under a person’s conscious control, to anything like the degree which most people assume. This video contains some fascinating information on this topic.
Just one example occurs at about 10 minutes in. GLP-1 medications are big news at the moment. The video talks about the difference in how much natural GLP-1 the body produces in response to a meal in lean versus obese women. When the meal is high fat and low carb, there is no difference between the two groups. But when the meal is high carb, and particularly high in high glycemic carbs, then the obese women produce markedly less GLP-1 than the lean ones. GLP-1 produces satiety, stops you eating, and stops you wanting to eat. And depending on the choice of food, SOME people are simply not satisfied by a meal which leaves OTHER people satisfied for many hours - even though both groups ate the same and should have enough calories on board to carry them through. So when the first group get hungry and eat again, they are not being greedy or indisciplined, they are responding to biological hunger, albeit inappropriate. And once you are in a pattern of eating high glycemic carbs, you are in a self-reinforcing routine of overconsumption.
Now, if you are one of these people (I am), then once you know that about yourself, you can make the decision not to consume high glycemic carbs. Even having decided that, you have to fight against their highly palatable and addictive nature, and the fact they are ubiquitous and cheap and it seems everyone else is eating them. And you have to do things which some consider socially unacceptable, e.g. I just visited a 96-year-old friend on her birthday, and declined the sweet treats she offered. At one time I would have had one, because it would be rude not to. But then gone away with overwhelming urges to eat more of all the wrong stuff. And I’ve no doubt some people will think I’ve been self-obsessed or antisocial to do what I did. But then no-one with any sense would judge an alcoholic for declining a drink, would they.
And that is only one factor at play, there are many others.
None of this detracts from the fact that T1D is a different disease, with a different aetiology. Or of the trauma of living on a knife-edge trying to manage it, with no respite.
It IS possible to undo the worst excesses of obesity. It is. It is extraordinarily demanding, and most people need support to do it. They also need accurate information about HOW to do it. And “be more self-disciplined” is pretty useless.