The consuming desire of most human beings is deliberately to plant their whole life in the hands of some other person. I would describe this method of searching for happiness as immature. Development of character consists solely in moving toward self-sufficiency.

This quote means that many people want to rely on others to make them happy, which is immature and not a good way to develop character. Instead, they should work on becoming self-sufficient, which would lead to them becoming more individual and independent. Becoming self-sufficient means that you are able to do things for yourself and not depend on others to do them for you. You rely on yourself to make decisions and be responsible for your own actions. Becoming self-sufficient is the mature way to develop character and ultimately lead to greater happiness.