Mostly, a cat needs to be given the room to get comfortable and adjust to the smell of a new person or animal. It especially helps to let your cat smell something the new visitor has touched, so that they can get used to the new smell. That way the smell will be more familiar and less threatening to the cat. For more information on why cats hiss visit the source.Source: OMGfacts
When you try to get rid of fear or anger, what happens? You just get restless or discouraged and have to go eat something or smoke or drink or do something else. But if you wait and endure restlessness, greed, hatred, doubt, despair, and sleepiness, if you observe these conditions as they cease and end, you will attain a kind of calm and mental clarity, which you will never achieve if you’re always going after something else. ~Ajahn Sumedho