Why does my cats fur stand on end?

Are they always scared?

When a cat is relaxed, his fur is too. But if he feels frightened or intimidated in any way, a completely involuntary hair raising reactions causes ‘puffing’ of the fur on his shoulders and back which can also sometimes make his tail look like a brush.

The scientific name for this is piloerection, which just means ‘upright hair’. Triggered by the cat’s stress, a hormone cascade causes tiny muscles at the base of each hair to contract, lifting the fur away from the body.
Causes of this usually are; loud noises, new situations, meeting new people or animals, feeling cold.
Benefits of this reaction are that the cat can look more threatening and it can actually help him retain his body heat.