Windows are part of your outlook. Therefore keep it clean.
For it is mostly through glass windows, either closed or open, that you look out, observe, gather information, and make judgements. Therefore it is of first and paramount importance that you keep your windows clean.
Simply because windows stained with dirt can distort your vision and thereby your impressions and perceptions of people and situations.
Simply because many problems that you attribute to others are of your own making and caused by log in your own eyes1—the windows with which you see the world.
And why a third time?
Simply because clean windows allow the sunlight to brighten up the interiors of your home. Allow nothing to obstruct light from passing through, for light brings cheer and hope with it.
Let me conclude by saying that it is a good exercise for your mind and body and sometimes even your computers to clean up windows.
1Matthew 7:5 Sermon on the Mount, Bible.
“The eye is the lamp of the body; so then, if your eye is clear, your whole body will be full of light.”
— Jesus Christ (Matthew 6:22 NASB)