It depends on your culture. If you just live together & that is not accepted by your friend/parents ,you will miss out on a lot of support & that can cause unhappiness for some people.

Do you think all the married couple live happy life? Why there's so high rate of divorce ? I agree with Mukund: Happy or unhappy is up to you .

Actually it depends, how u look at life. U can be happy even without getting married. However, after getting married, u get mental satisfaction (if u get a best partner). U get a hand with whom u can climb on the ladder of happiness. Happiness lies in yr MIND. Go deep in yr mind, understand WHO u r. Once u have that ANSWER, u float on the waves of happiness.