Unlike rocks, we have a perspective.
Yes, from some fictional omniscient perspective, everything we do is just as pre-determined as actions of the rock, even if the internal interactions are lot more complex, but from our own, limited, perspective, even if we believe in determinism, we're still stuck with free will. Determinism doesn't relieve you of that burden.
To OP's point, however, as creatures of perspective, and reason, and driven by all the base biological desires that allowed us to become this complex through the myriads of forms over countless generations, we seek purpose, we create purpose, a tendency which is now integrated into the ever-evolving survival mechanism that brought us to this point.
If any of the innumerable factors that brought us to this point, be it one stray particle at the dawn of time, or one dividing cell billions of years ago, had failed to do so, we wouldn't be here to search for purpose. Thus, retroactively, every life, and every atom - every action and reaction - has been granted purpose, by us.
And similarly, your life, is part of a never ending chain of events in some future purpose, to be granted purpose by some distant future, perhaps, one day, god-like beings who will dominate the universe, and give it purpose.
Or maybe we'll fail to leave this rock, and die with the rest of this tiny fragile biosphere. On this time scale, it's not something you need to worry about beyond your ability to affect that long term survival, should you have any, beyond simply breeding.