That problem has been discussed extensively in philosophy. The answer has great implications for e.g. moral theory and the meaning of life. To know what has value is to know what is desirable, what we rationally ought to believe in. As always is the case with philosophical questions a satisfactory answer hasn't been found.
Man is supposed to be the crown of creation.
Because man is capable of creating thoughts and forming global communities, the most complex structures we know of. Atoms organize into molecules that organize into organisms that in the case of man organize into societies. This ladder of complexity seems to be a fundamental property of the universe we know. What we consider valuable is usually near its top. An animal is much harder to describe than the corresponding soup of proteins.