Can a solitary man be happy? Maybe. But if you are Aristotle, and you believe that "man is a political creature and one whose nature is to live with others" it would be strange for a happy man to live like a Hindu sadhu in solitary contemplation.
Some people, Aristotle notes, disagree. They say that the fortunate man doesn't need friends. But they are missing the point, at least in Aristotelian terms. For such a person measures other people by their usefulness. His friends are the second-class friends that are not founded on "goodwill" but pure utility or pleasure. Enough of him!
That is not Aristotle's way. For him, happiness, eudaimonia, is full human flourishing, and full human flourishing must mean friends of the best kind: other virtuous men full of practical wisdom that do the right thing for the right reason in the right way at the right time. A happy, virtuous man needs friends, not for what they can do for him, but for what he can do for them. For a happy, virtuous man will want to spread the wealth around.
And besides, a solitary man would find it hard to be "continuously active," says Aristotle, and since activity is in itself pleasant, therefore friends are needed to provide the occasion for activity. Not only that, but "a certain training in virtue arises also from the company of the good." Even in Aristotle's time, people had to "use it or lose it." And that goes for virtue.
And besides (again?), just as life for animals is perception, life for man is defined "by the power of perception or thought". Thus to perceive and think and see that we perceive and think, and to perceive that one lives is in itself pleasant etc., etc., and since life is desirable and pleasant so then will be the perception of the life of a man's friend, because all the happiness and pleasure of considering one's own life will apply to the consideration of one's friend's life, "and this will be realized in their living together and sharing in discussion and thought".
Oh good. So that is settled. You can see, as so often in Aristotle, the charming hidden agenda. The purpose of life is to sit around and philosophize with a worthy friend. So therefore the happy man, reaching his full flourishing, is a man with the capability and the virtue to philosophize wisely--with a like-virtuous and happy friend. Q.E.D.