Human beings are a noisy, quarrelsome lot and many people want peace and quiet. The U.S. is large, fairly wealthy, and mobile so most adults can afford to live on their own. If Aunt Sally keeps tormenting us about getting married, we can move 4000 miles away from Aunt Sally.
We discussed this in communication theory during my sophmore year in college. American and Europe are becoming individualistic culture compared to the collectivist culture most of the world lives in. As much as we want to believe people should just get some courage and walk up to someone to have a conversation this is not how people interact. We rely on getting introductions in order to make social connections with new people. The more you live along the worst this become as you become self obsessed, and your social skills to connect with other begin to deteriorate.