Inevitably and intrinsically, the ultimate objective and, as such, the ultimate end, of an individual human being will be to be happy. The question that Aristotle is asking is what are the required actions (and the required function) that lead to happiness. His answer is that these are all actions that are in accordance to human reason. With human reason also comes the necessity to act within certain extremes and to be able to find the appropriate middle way between these. The mean is also the 'right amount' out of each activity, action or enterprise that the individual sets himself about to complete. Aristotle exemplifies this with the action of being courageous: being courageous needs to take into consideration both the conditions in which the act needs to be completed and the threats that may arise from that action. The right amount...

Such a situation could be, for example, standing up for yourself in a heated conflict with other individuals. If the subject of discussion is worth standing up for, the individual could consider physical action in that sense. If not, it would be foolish and useless to undertake.
3. Glaucon's argument relies on the idea according to which justice is a good thing in itself, but a burdensome good because of the effects it can cause. An individual would be ready to implement justice and to follow a just cause not so much because one's own belief in justice, but rather because of the fear of the effects that may appear if justice is not implemented. In simple terms, you don't litter because you are afraid of the fine, not because you find littering itself a bad thing.