Elaboration
In its form as an emotional experience, a sense of life is a natural effect of a person's sense of reality. A sense of life arises from a person's subconsciously integrated sum of one's widest, most fundamental, implicit conclusions about reality, one's environment, the nature of life and oneself.
Because a person experiences it in part in the form of a feeling, that person's sense of life inclines one to act in specific ways. A person remains free to act in accordance with or contrary to one's sense of life inclination. But a hopeful sense of life involves an inclination to live consciously and to act to achieve one's values; and a depressed sense of life involves an inclination to remain passive.
Of course, most people's senses of life fall somewhere in between.
Note
1In my opinion, this definition benefits from significant inspiration provided by Ayn Rand's discussion in her book, The Romantic Manifesto.
