Why does Aristotle say that the happy person could never become wretched?

Aristotle says that happy man can never become miserable because he does not participate in mean or hateful acts.

If activities are, as we said, what gives life its character, no happy man can become miserable; for he will never do the acts that are hateful and mean. For the man who is truly good and wise, we think, bears all the chances life becomingly and always makes the best of circumstances, as a good general makes the best military use of the army at his command and a good shoemaker makes the best shoes out of the hides that are given him; and so with all other craftsmen. And if this is the case, the happy man can never become miserable; though he will not reach blessedness, if he meet with fortunes like those of Priam.