The context is human action and its "rationality":
[ 1113b.1 ] The activities in which the virtues are exercised deal with means. Therefore virtue also depends on ourselves. And so also does vice. [...] if it is in our power to do and to refrain from doing right and wrong, and if, as we saw, being good or bad is doing right or wrong, it consequently depends on us whether we are virtuous or vicious.
Human action must be "rational"; i.e. if an action is neither forced, nor performed because of ignorance, it is is "voluntary" performed.
actions of which the origins are within us, themselves depend upon us, and are voluntary.
In his discussion of courage [ 1115a.1 ] Aristotle distinguishes true courage, where someone remains at his post in battle simply because to do so is good and admirable [ 1115b.1 ] (the voluntary one):
Courage is shown in dangers where a man can defend himself by valor or die nobly,
from false forms of courage, like facing a disasters like shipwreck (the "forced" one).
The courageous man then is he that endures or fears the right things and for the right purpose and in the right manner and at the right time, and who shows confidence in a similar way.
[ 1116b.1 ] A man ought not to be brave because he is compelled to be, but because courage is noble.
[ 1116b.20 ] wild animals [...] are not to be considered courageous for rushing upon danger when spurred by pain and anger, and blind to the dangers that await them; [...] the form of courage that is inspired by spirit seems to be the most natural, and when reinforced by deliberate choice and purpose it appears to be true Courage.
[ 1117a.1 ] Nor yet again is the boldness of the sanguine the same thing as Courage. The sanguine are confident in face of danger because they have won many victories over many foes before. [...] When however things do not turn out as they expect, the merely sanguine run away, whereas the mark of the courageous man, as we have seen, is to endure things that are terrible to a human being and that seem so to him, because it is noble to do so and base not to do so.
We see here again Aristotle's stress on the "right middle" :
[ 1116a.1 ] The coward, the rash man, and the courageous man are therefore concerned with the same objects, but are differently disposed towards them: the two former exceed and fall short, the last keeps the mean and the right disposition.
In conclusion, courage is not "lack of fear"; the rash man falls short of fear, as well as the coward exceed in fear.
The truly courageous man is that able to "balance" fear and confidence.