Why is the rate of a reaction always positive and why? Why rate of reaction can never be negative? And why rate is an experimental derived expression?
No mystery. Chemists write equations so that reactants (on the left) yield products (on the right). So:
$$\ce{H2O -&gt; H^+ + OH^-}$$
and
$$\ce{H^+ + OH^- -&gt; H2O}$$

The reactions are read from left to right and reactivity is defined as positive from left to right - this means the reactivity for the reaction we want to look at is always positive. (great clarification from user Stian Yttervik.