There is an almost complete absence of natural philosophy in the typical philosophy curriculum. That is a mistake. Since the starting point for a common pursuit (wisdom) must be something common, it makes sense that nature be this common thing. This is especially the case in American society, where pluralism and diversity of cultural backgrounds make it impossible to find common ground in a common heritage. So human nature is what we have common. But because it is often difficult to distinguish in men between what is according to nature and what is according to custom, it may be wise to step back and consider nature more generally. If there are principles that must be true about every motion and every mobile thing, then they must also be true about men, at least insofar as they are mobile.
I'm just going to copy and paste of chunk of the Summa Theologiae (I-II.1.2) and rearrange it.
Every agent, of necessity, acts for an end.
For if, in a number of causes ordained to one another, the first be removed, the others must, of necessity, be removed also.
Now the first of all causes is the final cause.
The reason of which is that matter does not receive form, save in so far as it is moved by an agent; for nothing reduces itself from potentiality to act.
But an agent does not move except out of intention
for an end.
For if the agent were not determinate to some particular
effect, it would not do one thing rather than another: consequently in
order that it produce a determinate effect, it must, of necessity, be determined to some certain one, which has the nature of an end.
And just as this determination is effected, in the rational nature, by the "rational appetite," which is called the will; so, in other things, it is caused by their natural inclination, which is called the "natural appetite."
[Given that the italicized statement is true, the bold statements are true as well. Since 'end' has the notion of 'good', a natural end is equivalent to a natural good. So if man has a nature, he has a natural end and a natural good.]
The way St. Thomas proceeds in the following question is interesting: he follows a negative path to reach what constitutes man's happiness, namely God. This is interesting because negation of created things is proper to the consideration of God. Does this constitute a distinct argument for God's existence? If one denied the existence of God would it have to follow that men are necessarily unhappy? Or just not as happy as possible?