Suppose I have a function where the derivative if always positive. For example
$$f(x) = x $$
Then
$$f'(x) = 1$$
If I interpret $f'(x)$ as rate of change and it's always positive, how can $f(x)$ ever decrease?
What I mean by decrease is this. Suppose I'm at 0. If I move to 1 the function increases. If I move to -1 the function decreases. With a derivative that's always positive why the asymetry?
From your comment, it seems you're confused about the meaning of "increasing". When we say $f$ is increasing, what we mean is that $f(x)$ increases as $x$ increases; i.e. as we "move to the right".
If you want a function to increase no matter which direction you move in, you're in a lot of trouble: if you can increase $f(x)$ by moving to the right over some interval, then moving to the left over the same interval must necessarily decrease $f(x)$!