And believe it or not, that decision was shaped by your environment as well. Which is why most people give credit when credit is due.
It can also be a self-reinforcing behavior, as giving credit can also in turn benefit you later.
Again, just because everything is predetermined doesn't mean there aren't complex dynamics in motion, ones which greatly influence behavior and the chances of making a decision. Understanding these dynamics can also influence how you behave in the future.
Edit:
It seems that I am saying "if you kick a raccoon, it will run away". And you are saying "whether I kick the raccoon or not has already been determined".
In this case, my analysis is helpful/useful while yours isn't. My analysis can influence your future behavior (e.g. if you run into a menacing raccoon in the future, it will increase the chances that you will be able to defend yourself properly), especially if you didn't know this before, while your analysis probably doesn't affect behavior (except to promote apathy perhaps, which is likely even detrimental).
[ Disclaimer: I know nothing about raccoons, it was just an example ]
It doesn't mean you are wrong, it's just that saying "everything is predetermined" is not a useful way of looking at the world. It's like saying "white paint is white", which is not very surprising.
Strictly speaking, it's not even quite correct: quantum mechanics tells us that, even though the universe has (deterministic) physical laws, it also behaves randomly at a fundamental level. However, that does not change the fact that we don't have free will (since we have zero control over the randomness).
But it's conceivable that some of our decisions might be affected by quantum randomness (this is even obvious in some contrived, but realistic Schrödinger's-cat-like scenarios).