Aristotle argues that the supreme good for man is happiness. His rationalization is that the supreme good will also be the highest end, the end we pursue only for its own sake.
To arrive at the conclusion that the supreme good is happiness, Aristotle starts by looking at Plato's argument about the Forms, such as the Form of good, and sees that the Form of good is something that's good in itself while other goods are pursued for the sake of goods that are good in themselves. In other words, the Form of good is the final end result that other goods are used to achieved. Later, he defines the supreme good as a final end, an end that all other actions aim towards. Finally, he argues that we always choose happiness for its own sake and never for the sake of something beyond happiness. Happiness is something good in and of itself plus pursued for it's own sake, just like the supreme good. Therefore, happiness is the supreme good, as we see in his lines, "Happiness, then, is something final and self-sufficient, and is the end of action" (Bk 1, Part 7).
However, it's important to note that Aristotle is defining happiness as more than just the experience of pleasure. Happiness is a condition of the soul brought on by beneficial, or virtuous activity. What's more, no person can truly be called happy until that person has fully lived out his/her life, because only then can we see the final end result of all his/her actions as we see in his lines:
But we must add 'in a complete life.' For one swallow does not make a summer, nor does one day; and so too one day, or a short time, does not make a man blessed and happy. (Bk. 1, Ch. 7)