Too much, too often, will kill your mind of rational thought and cause you to pull inward to such a degree that you can lose who you are. There are many types of fears, rational and irrational. There are as many causes as there are thoughts in the Universe. It is natural to be afraid at times. How we deal with that fear is what is important.
Fear can be a very good thing.
It can save our lives. It is often instinctive and tells us when we are in mortal danger and must react to save our lives. But we can let it run away with us as well and this causes multiple problems. I was fortunate to be able to face two of my greatest fears a few years ago. My husband became a volunteer firefighter.