Happiness is not achieved by the conscious pursuit of happiness; it is generally the by-product of other activities.

Aldous Huxley (1894-1963) English novelist, essayist and critic

Essay (1943-01/02), “Religion and Time,” Vedanta and the West Magazine
    (Source)

Collected in Christopher Isherwood (ed.), Vedanta for the Western World (1945)

This quotation is sometimes misattributed to Huxley's "Distractions I" essay, from the same source (and collection).