The problem with delayed results. Let's say you have a smoker -they know smoking is bad for them, and maybe they would like to quit- if asked when, usually the answer is somewhere in the future and it's almost never: "now". They might say: "yeah I want to quit, but just not today".
We always want to do the right thing but we don't want to do it now -because now it's hard. So therefore procrastination is almost like a pain avoidance mechanism in the short term, at the cost of long-term permanent future pain. We trade future well-being for present satisfaction all the time.
This is so common because what you have to give up right now is perceived as direct immediate quantifiable satisfaction for a maybe, potentially, something good in the future.
The cigarette, the hamburger, or whatever the drug of choice, is seen as real. It's here holding a clear promise for satisfaction, while the future holds nothing clear, just a fuzzy hope for unrealized potential. Nothing is guaranteed.
This is where popular sayings like "a bird in the hand worths more than two in the bush." Meaning -don't give what you have right now, for something that you might never get in the future; did us more harm than good.
When it comes to your personal development that's exactly what you need to do; you need to give up the tangible for the intangible; give up the visible for the invisible; give up the real for the imagined. And this goes against everything that we were ever thought since childhood. No wonder so many people struggle to do the right thing.
Things can and should change. We can gradually wake up to the fact that if you do choose to resist the temptation, and you take better decisions over and over again -although it might feel hard and unpleasant at times- the future rewards are going to be a thousandfold better than any short-term satisfaction you might get now. The future could be a lot better, if you could just do better, now.