humans are only created to serve cats which are the true rules of this planet (see also sphinx), so we happen to love cat pictures :)

the other theory is:
up to 50% of the human population is infected by an cats based parasite, that only can reproduce in cats and is know to change the behavior of humans.
see here:
https://en.wikipedia.org/wiki/Toxoplasma_gondii
so either we humans are designed to serve cats from beginning, or cats happen to influence / control us through using their parasite. either way, we are trapped to click / up-vote cats pictures :)