Aristotle thought that happiness was the highest good because he felt that it was the main function of man. He realized that happiness is the highest good after he evaluated what chasing fortune, honor and logic was like. He felt that achieving these things meant living a good life because that was what formed a good living. He expressed how a person can accomplish happiness. He asserted that humans and animals shared biological and physiological processes with animals and even plants, like motion and awareness. However, humans are different from animals as in their potential to think rationally.
Simply that in order to really reach happiness in life, it is done through action learnt the growing process of life.