In Supp. Q65 A1, on polygamy, St. Thomas says:
All natural things are imbued with certain principles whereby they are enabled not only to exercise their proper actions, but also to render those actions proportionate to their end, whether such actions belong to a thing by virtue of its generic nature, or by virtue of its specific nature: thus it belongs to a magnet to be borne downwards by virtue of its generic nature, and to attract iron by virtue of its specific nature.In other words, God made his creatures such that they are able to fulfill their natural ends. But man has unique traits that distinguish him from the rest of creation:
Now just as in those things which act from natural necessity the principle of action is the form itself, whence their proper actions proceed proportionately to their end, so in things which are endowed with knowledge the principles of action are knowledge and appetite. Hence in the cognitive power there needs to be a natural concept, and in the appetitive power a natural inclination, whereby the action befitting the genus or species is rendered proportionate to the end. Now since man, of all animals, knows the aspect of the end, and the proportion of the action to the end, it follows that he is imbued with a natural concept, whereby he is directed to act in a befitting manner, and this is called "the natural law" or "the natural right," but in other animals "the natural instinct." For brutes are rather impelled by the force of nature to do befitting actions, than guided to act on their own judgment.Okay, now the sledding is getting a bit more heavy. I think that a reasonable summary for this is that man's fulfillment of his ends are guided by knowledge and appetite - or, better, by reason and will. This is in contrast to other animals, which are directed to fulfill their ends by instinct.
Therefore the natural law is nothing else than a concept naturally instilled into man, whereby he is guided to act in a befitting manner in his proper actions, whether they are competent to him by virtue of his generic nature, as, for instance, to beget, to eat, and so on, or belong to him by virtue of his specific nature, as, for instance, to reason and so forth [emphasis added].So by the natural law we are guided to eat, and to drink, and to reproduce - things which belong to us by our generic nature - and to do fulfill these ends in a proportionate manner, we are guided by by reason (which is part of our specific nature).
Now none of this has to do directly with the question of man's chief end, which is God. But that's because God is above nature, and we cannot attain to him by purely natural means. This is one fundamental reason - even apart from our sins - that we cannot attain to God on the basis of our natural acts, but only by his grace (as an aside, it's this sort of rather foundational principle that completely demolishes the ridiculous claim that Catholic teaching is Pelagian in any way: grace is the only possible means by which we can attain to God). But it has plenty to do with how we understand our lives here on earth.