My friend Kevin Myers says, "Most people are too insecure to give anything away." I believe that's true. Most people who focus all their attention on themselves do so because they feel that they're missing something in their lives, so they're trying to get it back.
Developing a giving spirit, as Menninger implies, helps a person to overcome some of those feelings of deficiency in a positive and healthy way. That's why Menninger says, "Generous people are rarely mentally ill people." A person is less likely to focus on himself if he is trying to help someone else.