How we cite our quotes: (Book.Chapter.Bekker #s); all Bekker line numbers are approximate, since they are keyed to the original Greek.
Quote #1
[...] for the complete good is held to be self-sufficient. We do not mean by self-sufficient what suffices for someone by himself, living a solitary life, but what is sufficient also with respect to parents, offspring, a wife, and, in general, one's friends and fellow citizens, since by nature a human being is political. (1.7.1097b7-11)
Here, Aristotle tackles the concept of the greatest good, which is happiness. It's the thing that motivates all human behavior and is, all by itself, perfectly satisfying and sustaining. But Aristotle's no fool; he understands that humans can't find happiness in total solitude because we're social beings. His declaration that a "human being is political" means that we are meant to live in community, interacting with and supporting the common good.
Quote #2
And so, in a word, the characteristics come into being as a result of the activities akin to them. Hence we must make our activities be of a certain quality...It makes no small difference, then, whether one is habituated in this way or that way straight from childhood but a very great difference—or rather the whole difference. (2.1.1103b21-25)
Aristotle speaks here about the necessity to raise a child up in way that will help him develop his "characteristics" (i.e. virtues). He believes that although we may be receptive to such an education from birth, we're not simply born with the virtues up and running. The task of such habituation falls not only to the family, but also to the political community and to lawgivers who enshrine desirable virtues in the laws that they write.
Quote #3
As for private expenditures, there are all those that occur just once (such as a wedding or anything of this sort) and anything the whole city or people of worth take seriously, as well as anything connected with the receiving and sending off of foreign guests or the giving and reciprocating of gifts. For the magnificent person is lavish not on himself but on the common affairs... (4.2.1123a1-5)
All virtues benefit the common good through the actions of good people. In this case, it's the magnificent person (one who spends a lot of his own money to improve the community) who sees to the things that ought to be done to ensure social stability.
Note that even private expenditures (i.e. weddings) have an impact on the community. Aristotle says that even the furnishings in the magnificent person's house have to be excellent, since it's his duty to stimulate the local economy and receive important civic leaders in style.