This quote is comparing everyday activities to important values like love and goodness. It is saying that the amount of slamming in an old screen door depends on how loud you shut it – meaning that small details can have an effect on the overall outcome. Similarly, the amount of slices in a bread depends on how thin you cut it – meaning that changing small things can affect the bigger picture. The quote then moves on to address abstract values – the amount of good in a day and the amount of love in a friend. It suggests that these values depend on how you choose to live or how much you give, respectively. In other words, the quote is encouraging people to be thoughtful and considerate in all aspects of their lives, no matter how small.