I'z in ur brein, kuntrolin ur thots

Evolution explains why LOLCats control your mind. The basic argument extrapolates on this little bit from a Yale study

Since keeping an eye on predators and prey was important during our evolution, Joshua New and colleagues investigated whether animals, both human and otherwise, are more likely to grab our visual attention. [...] As predicted, subjects were faster and more accurate detecting changes involving animals than inanimate objects. If experience were producing this bias, then people should also be good at detecting changes involving automobiles, which as drivers and pedestrians they have been trained all their lives to monitor for sudden, life-or-death changes in trajectory. Yet subjects were much slower in detecting changes to vehicles than to more rarely experienced animal species, indicating that learning is not the source of this difference. The bias for animals, the authors conclude, is like the appendix: present in modern humans because it was useful for our ancestors, even if useless now.

And the captions are one of these changes that people notice in pictures of cute kitties, which is why they catch our attention. That's the argument, at least.