I recently got a lesson in how one becomes a "cat lady." All my family members were out of town, and I was alone with four cats. It wasn’t my idea to have four cats, but I was the one taking care of them, which I had time for as everyone else was gone. Lucky for me, it was a temporary situation. It’s a small leap from being called a "cat lady" to "crazy cat lady," especially from people who don’t care that much for cats. But where did this stereotype come from? Cats have been domesticated since ancient Egypt, and even worshiped.
But cat ladies first got their reputation for weirdness in medieval Europe. This was a society in which a woman’s fondness for cats made her suspect. Before they acquired their demonic reputation, cats were regarded as soulless beasts. Some people claimed that they, and other animals, couldn’t feel pain. During the Middle Ages, when people wanted to make ruckus—during charivaris or other shaming rituals—they would often pick cats up and rip fur off of them in order to get them to make noise. It was considered good luck to burn cats, and plenty of people did just that in order to celebrate a major event or ring in the new year.
A lady who liked cats was going to be considered odd by the standards of the time.
Being an "odd" woman in medieval times could get one burned at the stake. Things have improved since then, but we still have the stereotype cat ladies. In fact, some cat lovers became quite well known for their obsession, which you can read about at io9. -via Nag on the Lake