A happy life is, I think, mainly about serenity, discipline, curiosity and the right kind of stimulation. Either you’re curious about stuff or you’re not, and “the right kind of stimulation” obviously means everything except drugs, alcohol and compulsive eating. Serenity has many ingredients, but I tend to define it as good enough, taking care of yourself, great theatre, soul-stirring music, nothing terrible or toxic, bills paid on time, healthy food, exercise, long walks in big cities, great cappucino, spirituality if you want it, even-keel relationships, et. al.
The problem for most people, I suspect, is that the kind of happiness they knew or at least occasionally tasted in their late teens and early 20s resulted from the riding of a special kind of spiritual wave with really close friends, good drugs, breathtaking sex, etc. This kind of life led at times to feelings of joy, ecstasy and even a form of transcendent satori, but it simply can’t be sustained when you embark on your solo journey to adulthood and have to start focusing and getting ahead and shouldering responsibilities.
Most adults aren’t fully honest when you ask them if they’re “happy”, but if they were they’d probably answer, “Well, yeah, mostly…I mean, I was truly happy at times during my sex, drugs and rock ‘n’ roll days but that stuff’ll kill you.”