Understanding might not be enough
Every emotion is like a treasure trove which is hidden in a dark cellar, one needs the appropriate contemplation to shine a light and reveal the specific feeling. If the individual understands how good something is then they will be aroused with a feeling of a desire for what they are contemplating. On the other hand if they are contemplating how awesome something is, then an individual will want to withdraw from the thing they are contemplating.
The more real in one’s life something is then the easier it will be to feel something towards the thing that one is contemplating e.g. if one likes pizza then just thinking about how good it tastes will probably cause a certain desire for it.
If the subject is more abstract though then the individual will need to connect with the concept and have a feeling for the idea e.g. someone who over eats, for such a person the reality, enjoyment or compulsion of eating is very real but the good of not eating as much is quite abstract to their particular mindset. Such a person might need help to correct the way that they look at their situation and their relationship to food. To explain and help the individual understand the negative consequences of over eating might be quite easy but that will not necessarily change their will to stop eating. The change in what the person does comes when their understanding can cause a feeling or emotional decision which is stronger than the desire to eat.
To be able to move forward the individual will need to feel good about where they are going and less emotionally connected to where they have been. If they don’t feel good about where they are going then what is pulling them towards change? And if they still feel connected/good about what they were doing, why would they not return there?
Achieving a change in one’s life sometimes can be as easy as reading a book (depending how real and achievable the desired good is in the mind of the individual as explained above), at other times though counselling might be necessary to help reach your goal.