Because it makes cats into junkies! The catnip plant is a member of the mint family, it contains an oil called hepetalactone, an unsaturated lactone which does for some cats what marijuana does for some people!

When a cat comes into contact with this plant it goes on a ten-minute journey of ecstasy.
Catnip (Nepeta cataria) is not the only known plant to cause your cat to flip! There is Valerian (Valeriana officinalis) too, which produces strange reactions in cats.