There are four types of love according to Lewis. They are Philia, Storge, Agape, and Eros. Philia is friendship love. It is the kind of love that would cause a man to lay down his life for another. Storge is affectionate love. This is the love that a mother has for her child and is most easily related to the kind of love that animals have. This love is completely necessary for survival. Agape is the love that must go throughout all the other three loves. Agape is the love that God and humans share. If Agape, or God's love, is not part of Philia, Storge, or Eros they will all fall apart. You love because you have experienced the love of God.
Eros is the romantic love that is shared between a man and a woman. Lewis is quick to distinguish between Eros and Sexuality (Venus), although he would most definitely agree that they are without a doubt connected. He even calls it humorous in that something as beautiful as Eros is connected with something like Venus.
Many mistake Venus for Eros. The quick test to find out if you have Eros with Venus or just Venus is to look at the five minutes after sex and look at the attitudes of both involved. If all that is there is Venus then "the carton will be thrown out." As Lewis says, you do not keep the carton after you have smoked the cigarettes.
Perhaps the most important point that Lewis makes in this chapter is that Love is a choice. The passion of young lovers fades over time if God is not a part of the relationship. It is important to know this now before you enter into a marriage only to find the passion to have gone away. If you know that you need God to begin with, then it is much easier to stay faithful and loving even after the "honeymoon period" has ended.