The Humanistic/Existential theoretical paradigm of behavior bases its foundation on the premise that all personality traits are biologically inherited and then molded through social learning.
However, when it comes to the "Unique versus Universal", then we need to focus on Maslow and how, in his opinion, the hierarchy of needs must be met in order to comply with all the factors that lead to self-actualization. Therefore, the first step to understanding the reason behind our reactions and behavior is to ask ourselves whether we have met all of our needs. Think about this: why would someone be consistently hungry? Because there is a need of additional nourishment either through plain food, or by adding nutritional back-up to the daily diet. Similarly, the reason why some people may consistently (sometimes compulsively) yearn for things, complaint about things, wish for things, or even resent things in their lives is because somewhere along the line they failed to recognize a need, or failed to satiate it.
When it comes to universality traits in a humanistic and existential theory, the main paradigm is that all humans are basically "good". The universal traits of the human race are explained under the OCEAN (OR CANOE/NEOAC) acronyms also known as the Big Five factor. The argument is that we are all basically agreeable, or basically open, conscientious, extroverted or neurotic. Those main five traits are what makes human personality "universal". Going back to the example of how we need to satiate needs, think about this: if you listen to your life and attain what you need to become content, you will not have any reason to be a behavior problem, or a neurotic. Instead, you may fall under the universal OCEAN category of "open", "agreeable", and "extroverted". Those who consistently meet the needs of others and fail to help themselves ultimately become "neurotic" and will display pathological behaviors. That is why it is so important to help ourselves first before we can attempt to help anybody else.
The "unique" aspect of personality under a humanistic and existential paradigm is defined by Allport's trait theory as "cardinal traits". These are traits that are latent within our behavior but only come out during extreme or special circumstances, such as a traumatic event, or as a defense mechanism. Again, the humanistic theory views these cardinal traits as learned behaviors that are mainly inherited from our ancestors (such as the proneness to anger, depression, bipolarity, or positivism) which we have molded to our unique needs under the parameters of our immediate environment. Therefore, if we use Maslow to define uniqueness as an example, let's take the case of someone who comes from a long line of passive-aggressive personality traits. Although this particular person will undoubtedly have a tendency to also show passive-aggressive behaviors, keep in mind that pathological behavior such as this is the product of needs that have not been met, such as safety needs, belonging needs and self-esteem needs. However, if an individual recognizes these patterns of behavior within and takes the steps to modify and correct the behavior, then the unique cardinal behavior of passive-aggressiveness is substituted with a more positive choice of action. This is how universality and uniqueness can both "cause" and influence each other.
The humanistic / existential theoretical model attempts to answer the most basic and elusive question of philosophy: what is the meaning of human life? French existentialists Jean-Paul Sartre and Albert Camus argued that life had no inherent meaning. They believed that the task of each person is to create meaning through art, beauty, philosophy, or virtue. In this theoretical perspective, the human experience is, in a sense, universal. Each person struggles with the awareness of his own mortality and the inherent meaninglessness of life. However, the resources and methods available to us as we seek to create meaning are unique. Those resources -- art, beauty, virtue, etc. -- are culturally specific, and vary across time and place.