Interesting question. One answer is that certain parts of our culture associate sadness or sorrow with the possession of a depth of character, and happiness with a certain superficiality, that is, with a character that only sees what is on the surface, or on what is most easily accessible to a person. This is in turn often associated with the material, whether it be how things look (beautiful cars, bodies, houses) or with what one can acquire with money (rather than with one's soul).
But there is another weighting one can give to happiness and sorrow, one that we find in several places, including the Buddhist tradition. On this account, while it is true that sorrow or suffering is a fundamental aspect of the human condition, so that the person who experiences sorrow is more enlightened, shall we say, than the person who merrily goes through life without experiencing sorrow, it is a sign of greater enlightenment to accept the facts of the human condition that cause one sorrow (sickness, death, loss of various kinds), but to rise above these to a state of happiness, or at least ,tranquillity. On this account, in the end, happiness would be valued more than sorrow, for it is based on a fully complex understand of experience.