There are a few major reasons that people develop a fear of spiders, but the most common is that everyone else seems to be scared of them.
You've probably been told, since you were very young, that if a creature has 8 legs, that means bad news. Perhaps your mother or father had a spider phobia?
This understanding of spiders as bad and scary is not common for all cultures, but is rather usual for the Western hemisphere, specifically Britain and the USA. This may be because spiders are often portrayed as villains in children's books and then as monsters in horror movies. When you look at countries like Cambodia, where spiders are commonly eaten, none of the fear or cultural dislike of spiders exist. So really, you would not be afraid of spiders, if you never had the opportunity to fear them!
Another really common reason for people to develop spider phobias is all of the information surrounding spiders and what they can do to a person. Watching TV shows, you will be shown a lot of information about how horrible and dangerous spiders are. An example is the False Widow spider, native to the UK. Numerous headlines on how dangerous the spider was created panic amongst the population, but merely 3 months later, it was all forgotten.
Another possible reason why you may have a spider phobia is that our ancestors had absolutely no medicine or any antidotes to poisonous venom. And, with a lot of venomous spiders running about, if Grog got bitten by it, Grog would die! Psychologists believe that because of this knowledge, we developed a fear of spiders built into our minds and that people with spider phobia simply have a greater connection to their survival instincts. Bet you didn't imagine that.
The reason why it is important to understand what causes spider phobia, is because we can begin to change small aspects of our lives to deal with it. For example, if you watch a lot of nature documentaries about spiders that really overplay how likely you are to be attacked by one, try and reduce the amount of time you spend reading up on or watching shows that will increase your fears.