God, grant me the serenity
To accept the things I cannot change;
Courage to change the things I can;
And wisdom to know the difference.
That is the short version, but I give it as an example of two reasons answering the question why do people resist change. They are acceptance and courage. Mostly the acceptance comes with finally acknowledging that the only person one can change is oneself. This is learned from human relationships. People already know they can't change the weather, but accepting that they can't change another person can be difficult. Of course, fear is a big reason why the unacceptance takes courage to change.
Fear of failure and fear of success, fear of loss, fear of not receiving, and fear from bad previous change experience, are all reasons for resisting change. Some belief patterns are involved too. What of the person who believes he is not teachable anymore? Like the saying, "You can't teach an old dog new tricks?" That might be an acceptance for some, but others use it as a reason to stay in the same old same old. Lack of self-confidence resulting from a previous change with a perceived bad outcome can also cause resistance.
Sometimes people have total burnout from the effort to change, and resist the courage needed to change again. It often means; restructuring, reorganizing, reorienting, and the ability to adapt. Some flexibility is called for. Other times people don't see change as necessary. "If it isn't broken, don't fix it." When outcomes are unpredictable (which is pretty often), fear seems to rise, and people get stuck in the worry, and paralysis of resisting change.
Old habits may die hard, but the death of them is change. An example is, smokers who change to become smoke free, know they changed their behavior. Instead of enabling their habit, they changed their behavior. They know why we resist change, and accept the courage to change. For some it is a matter of perception. Changing the perception as a smoker to a non-smoker helps to change reactions to responses. The smoking trigger begins to become undone.
Lastly, among the many reasons to resist change, is ignorance. Ignorance means to be unaware, not stupid. Men and women resist change because they are unaware of the choice to change, the okayness to change, the possibility to change. We are ignorant of the change we make when we accept responsibility.
All in all, it takes acceptance and courage to change, and there are many tools available to help.