Agree with PPs that life itself is amazing. I had an unhappy childhood and hated high school but from the age of 16 have been very content.
From 16- 24 I guess studying made me happy and various groups of friends at college and uni as well as part time jobs and the social life that facilitated. Also house parties, travelling, dancing in indie clubs, waking up in the morning at Glastonbury on a gloriously sunny day after watching amazing bands. Youthful activism and seeing possibilities in the world and relationships which ran their course and ended on (mostly) good terms with lovely guys who helped me learn about myself.
Life has been a series of stages and most (post 16) have been very happy for different reasons. Teens and twenties - great as outlined above, then late 20's and early 30s when I moved to London, met DH, went to lots of gigs and galleries, had lots of boozy work nights out and nights out with new friends was also great. 5 years enjoying the capital pre-kids which was fab.
Then early 30's to now (40) has been about DCs, DH family holidays, home life, staying in, cosy evenings with board games and better wine than I could afford in my 20's, reading, box sets, only occasional gigs and theatres trips but that's all I want or can be bothered with now!