The window is associated with seeing. It allows me to see outside. It allows me to see inside. It is obvious why the window was associated with seeing and understanding. Therefore we can talk about the windows of a person's heart, or a person's mind, or a person's behavior.
Windows provide insight.
Insight provides understanding. Understanding provides guidance. We are constantly looking for windows that provide insights into life. The Christian understands that Christianity is about life. He or she understands that the Bible provides insights into life.