Why do rabbits like grass?

Wild rabbits like grass because it's sweet, nutritious, and high in fibre. Pet rabbits like fresh grass too, but it's not as good for them as hay (dried grass), because fresh grass can have parasites on it, and dried grass is higher in fibre. Other from grass, wild rabbits eat other things high in fibre, like twigs and woody weeds; because pet rabbits don't eat these things as often as wild rabbits, hay is better for them than fresh grass. Hay should be available in unlimited amounts (just like grass is for wild rabbits) so that pet rabbits can "graze" on and off all day long (just like wild rabbits do -- grazing is a natural rabbit behaviour, and all pets are happier and healthier when they're allowed to engage in natural behaviours). See the related question below for more info about a healthy pet rabbit diet.