This quote is saying that it is not a lack of love and affection between two people in a marriage that makes them unhappy, but rather a lack of friendship. A successful marriage is not just about love; it is important for couples to be friends with each other and support one another. When two people have a strong friendship within their marriage, they are more likely to stay happy together. A lack of friendship between spouses can lead to misunderstandings, disagreements, and eventually, unhappiness in the marriage.