Why is the rate of a reaction always positive and why?
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.