Plato asserts that everyone always desires the good; that may mean either:
1) whatever we desire is good
1 allows for an extremely subjective understanding of "good", since one person may desire a fattening meal whilst another might desire a healthy meal. In fact, it allows just about anything to be considered "good". For this reason, Plato rejects 1, pointing out that people judge poorly.
Instead, Plato says that "good" can be understood as what which we can desire, a kind of form that we wish to achieve or attain. In other words, Plato is saying that we evaluate our desires, i.e. we're not purely appetitive; he thus makes a distinction between desires and urges, the latter being purely appetitive.
Thus what we desire the most is that which is best according to our judgement.
To desire is generally (i.e. where possible) to be motivated to do something. This is distinct from being moved to do something, where one feels one has no choice in the matter, and that one must follow a particular course of action.
If I judge something to be [good / best] I am [motivated / moved] to do it
This suggests a strong form of internalism, especially in the statement that one can be moved to follow particular courses of action based upon judgements of relative value. But how can one then account for the weakness of will? Plato could reply that when somebody appears to take the wrong choice, they simply make a bad judgement, but personal experience shows that to be untrue.
Since desire involves a judgement, and Plato held us to be, at least in part, rational beings, then one must accept that chosing the wrong option is an incorrect rational judgement based upon one's previous rational judgements of what is good / best. In other words, one may correctly judge which action is best, but one will then chose a different action.
One must change Plato's maxim to say that if I believe that action X is best, then I am either motivated to do action X, or I am irrational. One must remove the suggestion that one is moved to do anything. But then this itself seems wrong, since we are often moved to act upon a brief judgement without feeling the need to make further judgements as to which course of action is correct, e.g. saving a child from a fire. Am I always irrational if I believe that action X is best, and yet I am not motivated to do X? And would I be irrational for then not doing X?
It seems that one must either accept other motivations other than the results of rational analysis, or one must simply claim that we are constantly making bad, irrational choices, and that our will is weak.