Blackburn is so interested in suggesting that gratification accounts of
happiness are mistaken that its easy to forget what stands in their
favor. What is appealing about thinking about happiness in terms of
pleasure or gratification?
These are natural human needs that appeal to the human psyche and evolutionary desires of self propagation and selfishness. Happiness being rationalized in this way allows philosophers to support happiness as being fundamentally aligned with personal gratification as well as selfish self propagating desires. It would make sense that being happy would correlate with self gratification and doing something in ones own interest.