What do people fear the most? I've divided it into a few categories:
For many, it's the creepy crawlies: snakes, spiders, mice, and bugs.
For others, it's larger creatures, like dogs, children, and terrorists (or children who are terrors).
For some it's all things health related: doctors, dentists, shots, cancer, vomiting, even death.
For some it's social: speaking in public, crowds, a job (or lack thereof), people in general, divorce, relationships, and the economy
For some it's spatial: heights, closed spaces, open spaces, flying
For some it's natural: dark, storms (and the aforementioned creepy crawlies)
For some it's personal: finances, insignificance, rejection, failure
When it comes down to it, the primary motivations are avoiding discomfort, pain, and death. Even when it may seem irrational (1 in 9.2 million chances of getting killed on an airplane; 1 in 2.3 million chances of being killed by lightning).
One of the greatest draws of the Christian faith is that it gives the believer the chance to rise above all fear because death has no hold. There is nothing that can't be overcome with the power of Christ (who, incidentally, was the first to overcome death). And there is nothing (Romans 8:38-39) that can separate us from His love- whether we believe in Him or not.
So go forth, and fear not!