The way of the world.
Both of those rules could have been presented in terms of theological concepts--creation as a divine trust in human hands, human beings as reflections of the divine image--but our sources do not feel the need to do so. Some things just ought to be self-evident, the Rabbis seem to be saying, with no need to trace them back to first principles.
Another anomalous feature of derekh eretz in classical rabbinic sources is that it describes practices that are commendable but not formally mandatory. In a culture that values laws and does not shy away from enforcing them, it is striking to hear a practice described as "simply how it ought to be," without assigning a punishment to its violation. Calling something derekh eretz is rabbinic culture's way of shrugging in exasperated resignation and saying about someone's discourteous, impertinent, slovenly, or gluttonous behavior, "Of course he should know better, but what are we supposed to do?!" Even though Talmudic literature knows of a category called hilkhot derekh eretz ("laws of derekh eretz"), there are few offenses in that category whose violation carries a penalty.
You Have to Make a Living
One of the earliest senses of derekh eretz is one's livelihood. In the Mishna's tractate of wise aphorisms, Avot, we read that Torah is good 'im derekh eretz, "along with a worldly occupation," because engaging in both pursuits keep a person away from sin (Avot 2:2). Derekh eretz refers here to how one sustains oneself in the material world. That sense, especially as it appears in the dictum that "derekh eretz precedes the Torah" (Leviticus Rabbah 9 and elsewhere), underlies its most famous modern usage of the term. The 19th-century German rabbi Samson Raphael Hirsch who espoused a theology that has become associated with Modern Orthodoxy, called his religious approach "Torah with derekh eretz," using a traditional term to promote the idea that one's learning should engage both traditional Judaism and the secular world.