A distinction needs to made between a threat and fear. A threat is any force, entity or process that can impair the integrity of an organism or end its existence. Fear is an autonomic and bodily contraction that can be one response of higher animals to a threat. Fear has an affective component (like most bodily changes) and can properly be called an emotion. Anger is also an affective response to a threat characterized by a bodily expansion, and it too is an emotion. Any actual response to a threat will be driven by anger, fear, or instinct. In humans, love is also a possible basis for a response, but it is naive to believe that this is suitable for all threats or even a large fraction of threats.
In humans, with attachment and emotional needs as well as survival needs, threats can be infinitely complex. So complex, that it is understood that what seems at one moment to be a threat may in another moment is seen clearly not to be a threat. This cannot be overgeneralized though, there are some threats, even social ones, that are basic and cannot be reappraised away. Examples are abuse, bullying, humiliation, and betrayal. Still the point has to be taken that sometimes, the bodily response of either fear or anger is not needed.
Fear is realistic if the threat is strong. Fear has four stages in order of severity: alert, shrinking, freezing, and dissociating Fear might or might not engage the fight or flight system. Fear needs also to distinguished from defense, although in the animal world freezing is known as a defense. As stated above, most modern threats are social ones The best approach to such threats may be the ventral-vagal social engagement system which is compatible with both anger or fear (if neither is severe.)
It is a fashionable trend now in therapy and personal growth to encourage participants to "discuss their fears." What is really meant is discuss their threats. This naturally is wise as threats can be put in perspective and some will evaporate, and practical solutions to other threats will gel. However, in this trend it is implied that fear, apart from lethal threats, is actually a mental mistake. (And also, that anger is both a mental and a moral mistake.) It is proposed that social threats be 'understood' away. Fear and anger are both involuntary responses, and if the intellect is employed to discredit them, this is dissociation.
Fear arises in brain areas like the locus coeruleus in the brain stem and the amygdala in the limbic system. It is part of a 'threat detection system' that is much older than human consciousness. In humans, threat detection on this basis is of course potentially a great asset but it often becomes problematical because 1) young humans are very vulnerable for a long time, so being raised in an emotionally or physically unsafe environment causes a steady up-regulation of the threat detection system--it stays stuck on 'high'. 2) memory (past), imagination (future) and analysis can keep the threat 'alive' and so provide a self-perpetuating loop of fear, and 3) threats to the ego become equally or more potent as threats to the body, and if there is reliance on a false self, the ego is vulnerable 24/7. Rather than a time-limited response that arises with certain changes in the environment, fear can become a constant state, albeit with fluctuations in intensity and awareness. Baseline fear can be increase over time because of kindling. Once this fearful state is well established it is hard to cool the locus coeruleus and amygdala by thinking ('from above')--this usually just results in rumination. It is more effective to cool them 'from below', with breathing, neuro-muscular relaxation, or sincere emotional expression 'from the heart,' or 'from the gut.'
If one endures fearful threats past a point, the contraction becomes fixed. Young children are very vulnerable, and may be subject to threats that adults do not appreciate. The contracted state of muscles is stickier than the expanded state. Once contraction takes hold, in the absence of bodywork, it may be more or less permanent. However, the feeling of fear leaves the mind, but an interpersonal style develops that is vigilant and suspicious. The threats are being anticipated unnecessarily to 'make sense' of the body's condition.
For this reason, it is sometimes said that fear is living in the future (and therefore, again, a mental mistake). But the fear has a present basis in the state of the body. As long the body is contracted this way, cognitive work to debunk the sufferer's conception of what is the threat will have meager results and widen the split between the mind and the body. We naturally have an interest in combating fear through safety, either by a protected environment or by knowing what will happen. But safety cannot be achieved completely, especially for social threats.
A useful analogy is the comparison of a house cat and a sheep. The cat stays relaxed in the midst of activities and other creatures. Only when another creature comes very close and acts threatening does the cat react, usually by showing claws or taking a 'fighting stance'. The cat can run but it is not its first move. The cat immediately relaxes when the threat is far enough away. A sheep on the other hand, cannot defend itself except by fleeing or going unnoticed. Therefore sheep are always restless and on guard always for threats. Potential threats are fled before they are close enough to really evaluate. The point is that it is not anticipation, but the ability to fight that provides a sense of safety.
Now of course for humans' fighting may be unwise where there is the possibility of real bodily harm. But in the social arena, everyone can learn to fight manipulation, disrespect, shaming, humiliation, betrayal, being cheated etc. If one is prepared to fight these, it is not necessary to be constantly on the lookout for them. Chronic fear is closely related to suppression of feeling.