the "just" is doing a lot of work, too much. Most people with an addiction or weight problem are very well aware of what they should be doing (or not doing), but they'll be fighting with their body telling them constantly that they really, really need to eat or smoke. This is the hard part and telling someone "just do it" is not helpful in the least

That depends on your definition of "reasonable". Every single person I have met who is struggling to lose weight is attempting to consume fewer calories. Telling them to "just eat less" provides no novel information or actionable strategies. At best it makes you sound like the kind of person who most people avoid because you prefer being right to being helpful.