Intimacy is often a scary endeavor for many people. A fear of intimacy is not a fear of being close to someone, but rather, a fear of vulnerability and exposure.
I believe we often protect ourselves by placing boundaries so rigid that the ability to connect with another person is stunted.
The desire to connect with other people is a central part of being human. However, knowing how to develop intimacy and having the desire to do so are two different things. Understanding the root reason people struggle with intimacy, can help us understand the difficulties one has in developing intimacy.