When a person is emotionally unavailable, it means he isn't connecting with his partner on an emotional level. Such people don't share their feelings, and they don't place a great deal of value on feelings shared with them. There is a barrier between their emotions and the emotions of others, meaning they have an inability to share and to really be a part of a relationship.
Emotional unavailability can be a temporary situation. It can be caused by breakups or breakdowns of relationships that were important to the person in the past. For instance, if someone had a deep, meaningful relationship for many years and then that relationship ended, it can leave a lot of hurt and anguish to get over. The person will have to deal with her emotional pain and get herself back in order before she can open up to another relationship in a healthy way.
Children learn behavior from adults, and the adults who care for a child set the example. If someone grows up in a home where the parents are emotionally unavailable for the children, then there is a chance the children will take up that behavior. It may become a way of life that, unless caught and altered, will persist into adulthood.
Taken to an extreme, emotional unavailability is a psychological disorder. Sociopaths are individuals who do not feel emotions in the way that normal people do. Guilt and love are noticeably two things these people simply do not feel. People who suffer from this disorder may not even have the capacity to understand other people and thus the ability to really share emotions is not realistically possible.
People who are afraid may retreat back into themselves emotionally. If someone is afraid to take more risks with a relationship or if this person is afraid his partner will hurt him, then that person may cut himself off emotionally. If fear and distrust can be overcome, though, that person may become more emotionally available.