Since I have been living under the earth, I don't really believe that there is any reason we should be unhappy. Being happy is essential in life and the need for us to live happily is wise since we don't know when death comes.
I don't think that there is any reason we should be sad as life is one we should aim to enjoy and not to be unhappy about. There is a saying that he who got life has hope of better things in the future. Let's chose happiness over sadness.