I understand that, as one climbs the critical line, $\sigma = 1/2$, with increasing $|t|$, the non-trivial zeros of the Riemann zeta function $\zeta(s)$, $s = \sigma + i \, t$, become ever more frequent. The number of zeros in between zero and $T$, $0 < |t| < T$, is roughly proportional to $T \, {\rm log} \, T$, so that the mean distance between these non-trivial zeros is ever decreasing.
I'm interested in the reason why is that so. I'm not interested in observations, as there are many available out there, what I'm interested in is the root cause for this density increase.