Perhaps verse 18 could be rephrased as, "It is not good that man be independent." Our God establishes principles and patterns in His Word from which we can extract wisdom, the practical application of truth. Some of the most basic and fundamental patterns for His purpose are established very early in Genesis.
What is He showing here? That, in relation to God's purpose, the most and the best will not be produced in us if we are alone. If we are independent, we remove ourselves from the circumstances that will produce the most toward His purpose. In this specific context, God is not commanding everyone to marry, but He is clearly showing that marriage is better than remaining single.
Everyone understands from his own experiences that the more people who comprise a unit or community, the greater the number and intensity of problems. This occurs largely because our carnality drives us to compete rather than cooperate. Sometimes a person desires so strongly to be independent of this kind of community relationship that he separates himself in order to be completely free from the suspicions, distrust, offenses, and other hardships that occur within a group. To put it another way, it is very similar to a soldier running away from the battlefield to protect himself.
In its rawest form, it is selfishness and self-interest. It can be a self-serving avoidance of being useful, of contributing steadfast strength and encouragement, of being a right example to others or of being found wrong and corrected. If nothing else, we are detaching ourselves from the unit to which God intends we show allegiance and give service.