Fear is caused by a gap in knowledge. When we are uninformed or lacking in understanding we have an open spot for fear to fill. Depending upon our current emotional, physical, mental, and spiritual energy and comfort levels the fear may be easily ignored or handled. Or it could be something that completely derails us until we overcome it.
The place, it seems to me, that it is easiest to generate new gaps in our knowledge is through relationships with other people. Friends and family all interact with you time and time again, creating a continuous experience. Even without actively trying to set expectations you begin to count on certain behaviors, certain responses or attitudes. A change in what is expected can be very difficult to absorb without also taking on some fear.
It is easy enough to question the other, to try and fill the gap with knowledge thus displacing the fear. But questioning from a place of fear is hard to do without transmitting your fear to them - usually in the form of anger or hurt. So we tend to sit quietly with our fears hoping that they will go away on their own. Occasionally time does bring understanding and knowledge to replace to fear, but not always.
Courage is not the absence of fear. Courage is the acceptance of fear.