There are many things that we can be scared of or that bring about fear in us all in some way. What most people overlook however is that all fear actually comes from the same place, and is for the same functional reason. There are positive and negative consequences of fear, however fear is still a useful evolutionary tool in human survival.
Just as animals learn to fear larger predators, and very often mankind, humans in turn fear things which could be potentially dangerous or harmful to them. This is all basically from instinct to survive and fear of death that all humans are inbuilt with. The most common things we are scared of are likely things that in ancient times in human development were dangerous to us.
Spiders for example would have been feared by man for thousands of years from our origins in Africa because they can be poisonous and can kill us. They also live in close proximity with humans so aren't always easy to avoid even as a lot of things we aren't as afraid of as much,but that are more dangerous, such as lions or tigers are. There are millions of people with this fear of spiders even today, even in countries where there are no dangerous spiders at all. Whereas most people aren't inherently afraid of lions or tigers as much because they are easier to avoid, and wouldn't have been in close contact with man for hundreds of thousands of years for a fear to develop.
These fears we do have however are actually useful to our survival, despite not feeling very good at the time. As a simple example anything you are scared of inherently, you are more likely to avoid contact with, and so you avoid the potential dangers associated with whatever it is. This in turn makes it more likely we as individuals survive, and aren't killed by the dangerous things of the world.
The second way fears protect us is by giving us a massive adrenaline boost when we do chance an encounter with something we fear, provoking a fight or flight response, meaning we are better able to escape or to attack whatever it is we fear, again increasing our chances of survival should we ever have to fight something that we fear.
The fact that we still have fears today and that they are still useful is testament to their being a good and not bad thing to have. There are a lot of seemingly irrational fears around today, but these likely are associated with something that was or is still dangerous, and so even these are a good thing.