Prof. Martin Seligman, quoted in The Guardian:

There are three paths to happiness: the pleasant life, the good life and the meaningful life.

The pleasant life is what most of us think of when considering whether we are happy from moment to moment. There are short cuts to this - such as taking drugs or playing mindless computer games - but sooner or later most people look in the mirror and ask: 'Is this all there is?'

The good life comes through deep engagement in work, family life or other activities. In my case it is writing and playing tennis, but it could be any activity that one finds challenging and rewarding.

... the meaningful life - devoting oneself to an institution or cause greater than oneself. In a now classic exercise Seligman calls 'Philanthropy versus Fun', psychology students in one of his classes undertook to engage in one pleasurable activity and one philanthropic activity, and write about both. The results, he claims, were 'life changing'. The afterglow of the fun activity (watching a film, eating ice cream) paled in comparison with the effects of the kind action (volunteering at a soup kitchen, helping at the school fair). The reason, Seligman suggests, is that kindness is a gratification. 'It calls on your strengths to rise to an occasion and meet a challenge. Kindness is not accompanied by a separate stream of positive emotion, like joy; rather, it consists in [sic] total engagement and in the loss of self-consciousness.'