ARISTOTLE: "Now we call that which is, in itself, worthy of pursuit more final than that which is worthy of pursuit for the sake of something else, and that which is never desireable for the sake of something else more final than the things that are desireable both in themselves and for the sake of that other thing, and therefore we call FINAL, without qualification, that which is ALWAYS desireable in itself and NEVER (desireable) for the sake of something else.
Now such a thing, HAPPINESS, above all else, is held to be. [i.e. Happiness is always desireable in itself. Happiness is never desired for the sake of something else. eg. No one says "I desire happiness to get money.!" Rather, they sometimes say/think/believe:- "If I had more/(lots-of) money, I'd be happy/(ier)!"]; for this we choose always for itself and never for the sake of something else, but honour, pleasure, reason and EVERY VIRTUE we choose indeed for themselves (for if nothing resulted from them we should still choose them), but we choose them also for the sake of HAPPINESS, judging that by means of them [honour, pleasure, reason and every VIRTUE] we SHALL be happy. Happiness on the other hand, no one chooses for the sake of these (honour, pleasure, reason, virtue), nor, in general for anything other than itself.
...Presumably, however, to say that happiness is the chief GOOD [desired thing] seems a platitude and a clearer account of what it is, is still desired. This might perhaps be given if we could first ascertain the function of man. For just as for a flute player, a sculptor, or any artist, and, in general, for all things which have a FUNCTION or ACTIVITY, the GOOD and the 'WELL' is thought to reside in the FUNCTION, so would it seem to be for man, if man had a function. Have the carpenter, then, and the tanner, certain functions or activities and has man none?
Is he born without a function? Or as eye, hand, foot and, in general, each of the parts [of a man KB] evidently has a function, may we not lay it down that man, similarly, has a function apart from all these (parts)? What, then, can this be? Life seems to be common even to plants [So men and plants have a common FUNCTION in being alive. Hence being alive is not a peculiarly human FUNCTION. KB], but we are seeking what is peculiar to man. Let us exclude, therefore, the life of nutrition and growth [i.e. Two life-functions of plants.] Next there would be a life of PERCEPTION, but it also seems common even to the HORSE, the OX, and EVERY ANIMAL. There remains then an ACTIVE LIFE of the element that has a RATIONAL PRINCIPLE ...[... and if any ACTION is WELL performed, when it is performed in accordance with the appropriate EXCELLENCE; if this is the case,] human GOOD turns out to be activity of soul in accordance with VIRTUE, and if there are more than one virtue, in accordance with the best and most complete.
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. Let this serve as an outline of the good, for we must, presumably, first sketch it roughly and then, later, fill in the details." [Nichomachean Ethics; Book I, Ch. 2. 1097a line 30 through 1098a line 21]
Since all the virtues are ACTS whose FUNCTION is in accordance with a RATIONAL PRINCIPLE, it follows that the exercize of such virtues pleases our minds and tends to make us happy. Thus the old "saw" or "platitude" that virtue is its own reward. However, vice also has an immediate "reward" in the pleasure or gratification of what Aristotle would call a vicious desire. That is why he said above that he was only "sketching" an outline and would go into more detail -- which he does, in detailing, arguably, the most complete list of virtues and vices any philosopher ever "detailed". The quote, above, was from Book I and there are 10 Books in the Nichomachean Ethics.
And since human happiness, according to Aristotle, involves the exercize of excellence in accordance with a rational principle, happiness is not something that man and non-rational animals can have in common, CONTRARY to what one of your other responders wrote, in supposedly "interpreting" Aristotle's Categories.
Other animals may be content or satisfied, or miserable and unsatisfied in their immediate desires. But they are never either happy, or its CONTRARY attribute, which is sad because both happiness and sadness involve a rational principle --- peculiar to human beings with rational natures.
Aristotle Nicomachean Ethics