Like the fear of snakes, the fear of spiders is the result of thousands upon thousands of years of human evolution. Historically, these animals were major cause of death for humans. So it's simple: the humans who weren't afraid of snakes and spiders died and did not pass on their genes, and humans who had that natural fear survived and passed on their genes. Natural selection favoured the humans who were afraid of spiders. In other words, it is difficult for people to control their fear of spiders because it is hardwired into the human brain.

We've even fused with bacteria it seems like. I mean, the mitochondrion is a good example of that; it has a membrane and its own separate DNA, which is why it's believed to have originally been a bacterium which eventually became a very necessary subcellular component.

I was never afraid of spiders until I moved to the south and saw a brown recluse on my dog's kennel. I'm still not afraid of them in general, but the fact that recluses are out there, possibly near my bare feet outside, is not too pleasant.

In the south I would imagine you mean the "Fiddleback" which is extremely dangerous. Up here we have "brown recluses" as well, but while they are necrotic, medical treatment, even it's not terribly timely will easily save anyone bitten (unless you were already on the edge of death, otherwise). My understanding is that a Fiddleback bite can pretty much kill anyone, no matter how healthy, if you're merely unlucky enough to be sensitive to it at all.