Under the humanistic/existential theoretical perspective, personality is defined as the result of a series of processes and dynamics aimed to satiate the myriad of physical, emotional, psychological, and ethical needs that we require as complex, living and thinking beings. It comes to no surprise that the view of a stable versus a dynamic personality changes considerably under a perspective such as this one.
Humanistic theorists such as Abraham Maslow offers that human needs begin with the most basic and bare-bone necessities ranging from food, to shelter all the way to sexual activity and homeostasis. However, that is only the beginning; as humans meet all of their needs they require others, namely, needs that begin to move away from the physical realm and enter into the more sophisticated, philosophical and even spiritual fields. The reason for this is because theoretically humans change "as they go" once the most primitive needs are satiated.
This being said, consider the implications in the debate of stable versus dynamic. According to humanistic theory, humans are expected to change in terms of what they need so that they can develop into what Maslow calls the "complete human". However, what happens when humans simply do not change?
This is the definition of a stable personality. It does not necessarily mean something negative that humans do not change, after all, specific behaviors may work as defense mechanisms, or as safeguards for many individuals. However, when the inability or the lack of desire for change comes from a pathological source, then this may mean two things: a) the needs of the individual have been met in their entirety (which is highly unlikely), or b) the individual is too afraid to change for fear that future needs are not met.
An example of a static personality (a negative static, that is) would be a man who has had a full, healthy life but refuses or is too fearful of entering a committed relationship. According to humanistic theory, the last step of the ladder of needs is self-actualization, which includes the want to belong, to be loved, and to love someone. When an individual does not want to comply with this, or feels that this aspect is not needed, it is to be assumed that there is a negative motivation behind this and that at some point one of th essential needs was not met. Any inability to adapt is indicative of a type of trauma in meeting basic needs, according to theory.
A dynamic personality, however, hardly finds a negative connotation because of the tendency for dynamics to accept challenges, to tap on their problem solving skills, and because of their high level of adaptability. Therefore, someone who has a natural tendency to accept change, become flexible and embrace challenges is someone whose level of self-actualization is quite heightened.
With self-actualization comes creativity, spontaneity, and problem-solving skills that make the individual enhance their self-esteem. Therefore, the humanistic approach is all about the fulfillment of needs so that humans become less dependent on repetitive and safeguarding behaviors and, instead, become more willing to try new things and grow in many different directions.