Why do cats roll on catnip?

Cats roll on catnip because it releases the drug called Nepetalactone. This causes the cat to have a "high" sensation. This high lasts for 10-15 minutes, and is a safe natural high for the cat. (No need to worry). Cats know when they have had enough and move on. Some cats will not even go near catnip.

For more information about catnip check out this infographic: http://www.catnipsum.com/sciency_stuff.html