You weren’t born afraid of spiders. At some point, something happened and a learnt association was formed.
It could have been something that happened to you. Or it could have been seeing someone else afraid.
Whatever it was, a neurological link was created. Connecting spiders with fear & danger.
When you see or think about a spider that neural pathway is triggered. Activating the release of Adrenaline into your body. What you experience is the fight or flight response.
However, because it’s a learnt response it can be changed.