The eye is the lamp of the body. If your eye is well, then your whole body will be filled with light. But if your eye is clouded, then your body will be filled with darkness, and the darkness that takes over the body - if this is how you "see" - then it is a deep darkness.
In reading the Greek, I imagine this is a metaphor like seeing the glass half-full or half-empty. The "cloudy" or "unwell" view can keep one from seeing positiveness and possibilities. Could this be about pessimism which can lead an entire body (soma = life in the Greek) down a "dark" path where nothing is ever good or good enough?
If so, then having a "clear" eye makes it possible to see all opportunities and make choices which can enliven and enlighten a whole life: for a lifetime, including a whole/well life.
While much is said about having a positive mind set compared to having a negative one, Jesus makes it even more clear that attitude is everything with this metaphor. It will affect an entire life. What is our attitude about life? Do we see clearly? Or is our vision clouded with the negativity of the day or of my mind?
Lord, give me your eyes!