Often in physics we model something using proportionality constants. When we can intuitively say that some quantity grows as another quantity grows, we might assume that the relationship is linear and that the quantities correlate somehow as follows: $$a = kb$$ Where $a$ and $b$ are quantities and $k$ is a constant number.
Now we can measure some pairs of $a$ and $b$, and divide $a$ by $b$ to get the constant $k$. We can then proceed to compute other unknown values of $a$ by multiplying $k$ with other $b$ values if we can indeed assume that the correlation is linear.
But when quantity $a$ seems to decrease as $b$ increases, we often assume a different model:
$$ a = k/b $$
Why do we often tend to assume that $a$ is proportional to the inverse of $b$ instead of there being a negative $k$?