What did Aristotle mean when he said that "man is by nature a political animal"? To what extent is his theory justifiable?

In his treatise Politics, Aristotle argues that "man is by nature a political animal." What he means is that, by nature of his abilities, man is intrinsically poised to exert his influence on society. The specific abilities Aristotle refers to are the power of speech and moral reasoning.
Aristotle maintains that, of all the animals, man is the only one capable of coherent speech. When combined with his moral reasoning, man must use his speaking abilities for the benefit of humanity. Aristotle argues that each individual is a partner of the city-state. He maintains that nothing could be more natural than this partnership.
After all, the state (as a whole) is made up of many parts (countless individuals). To Aristotle, anyone who decides against partnership with the state decides against humanity altogether. Such a person, Aristotle contends, must either be a "lower animal" or a "god."
Why does Aristotle argue for this partnership? First, he sees the state as the chief proponent of law and justice. Essentially, Aristotle sees the political system as a vehicle for bringing about good in the world. He asserts that justice is an "element of the state." He is confident that "judicial procedure" is an adequate means of regulating the "political partnership" that he sees as necessary to a well-ordered society.
Aristotle sees man as a political animal because man is intrinsically equipped (through the powers of speech and moral reasoning) to benefit humanity through a partnership with the state.
http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.01.0058%3Abook%3D1%3Asection%3D1253a