One of the reasons God rested on the seventh day was to celebrate His creation. One of the major reasons for the Sabbath being established was so that the Jews would remember there is a Creator because when people forget this truth bad things happen.
For instance, check out what the Bible says in Romans 1:24 through the end of the chapter. There is a high price to be paid when we deny that there is a Creator and refuse to worship Him. The price is that we stop living like humans.
If you don’t have a guide book or a model of what something is supposed to be then anything goes. Many have rejected the guide book, the Bible, and have started on the path of relativism. For those people there is no clear, universal definition of what a human is to be and do. You are left with a guessing game and whatever seems right philosophy.