If you fail to plan, you are planning to fail - Ben Franklin
Show me someone who doesn't keep or even make New Year's Resolutions and I'll show you someone who will not follow plans and much less, often resists even making them.
If most people would agree with Franklin's quote at least intellectually, why then do they resist following a plan and in many cases even making one?
I refer to this as "lacking objective constancy." That means the inability to plan and remain true to that plan to reach an objective.
What underlies lacking objective constancy is lacking object constancy. That is a term well known to nearly all mental health professionals. According to Wikipedia, "Object constancy is a term coined by Margaret Mahler that is similar similar to Jean Piaget's object permanence, describes the phase when the child understands that the mother has a separate identity and is truly a separate individual. This leads to the formation of Internalization, which is the internal representation that the child has formed of the mother. This Internalization is what provides the child with an image that helps supply them with an unconscious level of guiding support and comfort from their mothers. Deficiencies in positive Internalization could possibly lead to a sense of insecurity and low self-esteem issues in adulthood."
When a person lacks object constancy, they are unable to internalize the caring of others and thus feel very anxious and insecure when those caretakers are not physically present. When that happens they are overwhelmed with a restlessness that often supercedes their being able to focus on tasks at hand and totally eclipses thinking of longer term objectives.
In essence these people are driven much more by emotion than logic and trying to keep those internal emotions of anxiety from overwhelming them and turning into panic. That is why when that external object (parent in childhood, partner in adulthood) is away, they often resort to compulsive behaviors such as drinking, eating, shopping, gambling, pornography, excessively rigid exercise and diet regimens. By doing so they are seeking to bind their anxiety and keep it from turning into panic.
Therefore one of the reasons many people don't keep their New Year's resolutions is that they don't know how they are going to feel in the future. And if their feelings change in the future so too will their commitment to whatever they are doing.
A second reason people don't keep or even make New Year's resolutions is that they believe that they won't keep them and in so doing they will then either disappoint others causing embarrassment or disappoint themselves causing shame.
A final reason people don't keep or even make New Year's resolutions is that because of their need and hunger for contact with their external object significant others they will drop whatever they're doing to appease, please, placate or conform to what the other person's immediate needs are. The reason for that is a fear of disappointing that other person and having them become angry or withdraw their affection and connection. And when the other person withdraws the connection to someone who lacks object constancy, that threatens to increase anxiety which can lead to panic.
One of the reasons why it's often easier to stay on course with a commitment by attending AA, other support groups, exercise classes or just walking with a friend is that having that supportive contact is a way of distracting a person from anxiety caused by a lack of object constancy.