There are a variety of reasons and the answer is not simple. I would like to introduce a different dynamic that also occurs which others have not yet addressed. Most people have been hurt in interpersonal relationships. With time and painful experiences, we all risk building up varying degrees of bitterness and becoming defended. This process begins long before we start dating, in our childhoods, when hurtful interactions and dynamics lead us to put up walls or perceive the world through a filter that can negatively impact us as adults. These adaptations can cause us to become increasingly self-protective and closed off. In our adult relationships, we may resist being too vulnerable or write people off too easily.
