Other than the possibly natural fears of falling and of unpredictably loud noises, children seem to have no inborn fears. Most all fears are learned. Why, for instance, are some children afraid of the dark? A small child is placed in the crib. She cries and seems afraid. You turn out the light and shut the door. Your child's reaction intensifies. You likely assume this is a result of turning out the light and shutting the door. But your child's reaction is a combination of your leaving the room and your child's being left in the dark. If you go back into the room, turn on a night light, and console her, your child settles down to go to sleep. If the night light is turned off, the fear reaction begins again. You turn the light back on and console your child somewhat until she falls asleep or nearly asleep. You then decide your child does not like to be in the dark and you leave the night light on. Over time, then, your child gets used to sleeping with the light on; this becomes a fear of the dark.
Had you merely gone back and consoled your child until she settled down and started to go to sleep, continued to rock or talk softly to her after the light was off, whether the light was on or off would have probably never become an issue. From your child's perspective, though, why do you make an issue of whether the light was on or off if there were really nothing to be afraid of? If you think it helps to leave the light on, there must be something about the darkness to fear. You have taught your child to be afraid of the dark.
Here, as with other developmental tasks, children vary in how they learn, how they proceed through the developmental task, and how they respond to the efforts of their parents to help them develop. The key for you is not to get into a struggle or authority conflict with your child over these normal developmental tasks. Using fear of the dark as an example, suppose you try the above steps but your child is still very upset when the light is off and just does not seem to settle and go to sleep. The solution is fairly easy. Give her a hug and leave the night light on. Come back to it from time to time, never making a big issue out of it. It is just something you and she talk about and work on now and then.