If nature is a person or like a person, then he, she, or it can be a source of ethical guidance. If nature is impersonal, then any ethical guidance can only come from someone or something unnatural or supernatural.
If the characteristics of a person are attributed to nature, as in “Nature wants you to be happy,” then the distinction between naturalism and supernaturalism is erased, and this is no different from theism, which is fine by me. But the naturalism of those who reject supernaturalism does not attribute any person-like features to nature. Nature, conceived this way, is not something that cares whether you are happy or not. It is not merciful or ruthless, nor even indifferent, since to call someone or something “indifferent” implies that he or she or it might have cared but doesn’t. Nature in this sense is not conscious or potentially conscious, and a fortiori has no thoughts, plans, or emotions. If there seems to be any consciousness, design, or purpose in it, that is either something imposed upon it by something or someone outside it, i.e., something or someone unnatural or supernatural, or else only an illusion that can be explained as the result of chance mutations, heritability, and natural selection. But then since any explanation implies a conscious being who discovers it, communicates it, or understands it, the explanation itself must either be an illusion or else something imposed on nature by someone who is outside nature. We ourselves cannot be a part of nature, as understood by those who reject supernaturalism, unless we don’t really understand anything. And, although undoubtedly there is a lot we don’t understand, there is no reason to believe we don’t understand anything. Hence the alternatives concerning ethical guidance stated in the first paragraph.