Such theories seek to find some essence that characterizes the behavior of things in some general domain of study: physical phenomena, life, the mind, language, and so on. Questions like "What is the mind" or "What is life" presuppose the meaningfulness of such a quest for general knowledge. We do assume that there is a nature of things, and we are led by the metaphysical impulse to seek knowledge at higher and higher levels, defined by ever more general categories of things.
A person will somehow seek for self-actualization. They say, in becoming self-actualized, you already stop seeking because you already have achieved a concept of the meaning of your existence. Thus, people get some good education, earn some money, build a good family and do something that will eventually follow the dreams they have been yearning for. And yet, some people discover later on that getting what you want could still not satisfy the questions about the meaning of their lives. People have to struggle to achieve these things and it is in their quest that they learn lessons about life. Every triumph and every failure adds something up to yourself and you always strive to become better the next time around.
Apart from our internal struggles in how we achieve the lives we want, there so many external things that also influence on how we live our lives.