Fear
Without it we wouldn't be around to long ..
A powerful survival mechanism fear plays a great part in keeping us alive but often what happens is that we concentrate on the physical attributes of the fear itself.Fear of Flying is good example of this.
Although most people know that statistically flying is far safer than driving. The person concentrates on possible disaster scenarios.
And our wonderful human brain must give us what more of what we concentrate on - it has no option.
So the neural pathways are reinforced and if we really work at it the thoughts become intrusive then we can start becoming obsessive.
The good news is that all fear thrives on being supressed and hidden. Once it is brought out in the open and discussed on both a rational and emotional level it starts to lose the irrational power that made so difficult to understand.
All a question of balance where fear is concerned we need just enough.
Habits
Often the original reason for the action has been lost in the mists of time.However, the repeated motor action has reinforced connections between the autonomic nervous system and associated emotional / physiological responses.
Smoking is an extremely good example of this. People often erroneously think they are addicted to nicotine.
(No Wonder when you think of the huge advertising budgets of the tobacco barons.)

The sensory homunculus, above, shows us how sensitive our sense of touch is in different parts of the body. Our hands end up enormous, as do our lips and tongue, because they’re so sensitive and have such a complex range of sensory nerves. image courtesey of Nina Jablonski
Apart from trashing our lungs: Smoking effectively utilises the most sensitive sensory nerves in the human body it's this that rewards the dopamine receptors in the brain and allows me to show clients how to keep the feel good factor they associate with smoking whilst losing the chemical poisons that are trashing their body.
So creating a good habit relies on repetition (to create strong robust nerve pathways)
Dismantling a bad habit is exactly that. Breaking down the motor action and replacing it with something more desirable.
Easy once you know how!