An article in tomorrow's Guardian explains:
The internet is one giant, virtual cat park, its foundation is one of cat fur, and its currency is the miaows of millions of kittens who lie patiently in wait for you on YouTube, Facebook and Twitter, acting like they pretty much own the place. These are indisputable facts, but nobody has ever been able to come up with a suitable answer to the attendant question: why? Why are there so many cats on the internet?
The problem is that they are asking the wrong question, which should not be "Why cats?" so much as "Why not dogs?" And the answer is that dogs are trying too hard. When a dog gets in a box or hides under the duvet or wears a funny hat, it is because he is desperately trying to impress you – longing for your validation and approval. When a cat does one of those things, it is because it felt like the right thing to do at the time. And it usually was. It is cool, and effortless, and devoid of any concern about what you might think about it. It is art for art's sake.