What do the following have in common?

• "Trying to lose weight? Just eat less and exercise more!"
• "Trying to get more done? Just stop wasting time!"
• "Feeling depressed? Just cheer up!"
• "Want not to get pregnant? Just don't have sex!"

Answer: they would all be quite successful if followed, but they are all difficult enough to follow that people who actually care about results will do better to set different goals that take more account of how human decision-making actually works.

If you eat less and exercise more then, indeed, you will lose weight. (I do not know how reliably you will lose weight by losing fat, which of course is usually the actual goal.) But you don't exactly get to choose to eat less and exercise more; you get to choose to aim to do those things, but willpower is limited and akrasia is real and aiming to eat less and exercise more may be markedly less effective than (e.g.) aiming to reduce consumption of carbohydrates, or aiming to keep a careful tally of everything you eat, or aiming to stop eating things with sugar in, or whatever.

People with plenty of willpower, or unusually fast metabolism, or brains less-than-averagely inclined to make them eat everything tasty they see around them, may have excellent success by just aiming to eat less and exercise more. In the same way, for many people "just cheer up!" may be sufficient to avoid depression; for many people "just don't have sex if you don't want to have babies!" may be sufficient to avoid unwanted pregnancy; etc.

But there are plenty of people for whom that doesn't work so well, and this is true even among very smart people, very successful people, or almost any category of people not gerrymandered to force it to be false. And for those people, the "just do X" advice simply will not work, and sneering at them because they are casting around for methods other than "just do X" is simply a sign of callousness or incomprehension.