Because cats are quite agile, and can land on their feet if they're not dropped from extreme heights, they were considered hard to kill. The myth developed that they had nine lives. No particular reason other than that.
According to a myth in many cultures, cats have multiple lives. In many countries, they are believed to have nine lives, but in Italy, Germany, Greece and some Spanish-speaking regions they are said to have seven lives, while in Turkish and Arabic traditions the number of lives is six. The myth is attributed to the natural suppleness and swiftness cats exhibit to escape life-threatening situations. Also lending credence to this myth is the fact that falling cats often land on their feet, using an instinctive righting reflex to twist their bodies around. Nonetheless, cats can still be injured or killed by a high fall. This was found on Wikipedia. Hope this helps!
I found out in my book from 3rd grade: It said it was just a myth. They say cats have "9 lives" because it was considered a lucky number! Cats are lucky they can take care of themselves! Plus almost all cats live above from 9 years, which is lucky too! But this saying was an old cat myth! It even has in on my kitten's dry food bag! It is called "9 Lives"! LOL!!
They seem capable of amazing feats--they are. They have the ability to make themselves almost flat, & toss a few off of a roof-they always land on their feet, they just keep coming back for more. You think that throwing them into the bay would be it, they swim better than dogs. Throw 1 in a wood chipper & they die-bad example. They're just very agile & are pretty tough-don't try those things mentioned-it was a joke