We may have very solid, and seriously unfortunate, reasons why many men do not successfully maintain contact with friends throughout our lives. Women seem to hold onto relationships more naturally. Those (often false) perceptions about what is and isn't natural may be the explanation.

Different Drummer: Mark Greene

Why Do We Murder the Beautiful Friendships of Boys?

References:

Remaking Manhood: The Modern Masculinity Movement: Stories from the Front Lines of Change

and 

Forbidden or Sacred Friendships