The eye is the lamp of the body. So, if your eye is healthy, your whole body will be full of light, but if your eye is bad, your whole body will be full of darkness. If then the light in you is darkness, how great is the darkness!
Matthew 6:22-23

How can light be darkness? The eye is a light for the soul. If it only lets in darkness by dwelling on evil, by interpreting the world through lenses of cynicism and pessimism, then it corrupts the soul, filling it with darkness, and then only darkness will come out. If only light is allowed in, then only light will come out. We can aid the light within other people by speaking more light into the world, creating a positive feedback loop.