While not agreeing
completely with all of her conclusions,
Carol Gilligan's
In a Different Voice is a good place to start in trying to sort out men's troubles in trying to comprehend the ways that women tend to relate to other women, to men, and to themselves, and how these differences often result in a very different but internally consistent
logic,
ethics and distinctly different approaches to issues like "
rights" and "
justice."
In a Different Voice is one of the founding texts of "difference feminism" and full of controversial concepts. Her analysis has been challenged by many, including other feminists and women's studies scholars. But the issues and notions raised are fascinating, and not easily dismissed, except by the careless (and the insecure).
In this context, Gilligan and many others would very likely say that the impression of a general friendship between women is a by-product of learning through experience that obvious divisions between women are very often used against them, and, as people whose social role has generally been that of emotional caregiver, there is a strong social pressure to conform to that role and at least appear agreeable and friendly, whatever one's real feelings may be.
So there is a tension, and many women learn to be disagreeable with each another in ways generally more subtle and indirect, ways that can easily be missed by a man who tends to deal with issues more directly, with "no punches pulled" as it were.
It's not that all women are friends, but that the consequences of being less than sociable were, for a very long time, (and in many cultures still are) quite severe if you were someone born to take on a woman's social role in life. To get along in many cultures you need(ed) as many allies, and as few outright enemies as possible.
Coincidentally, girls tend to develop verbal and social skills at an earlier age than is typical of boys, and the qualitative differences in their interactions are apparent to most observers, though measuring these differences in objective terms is not at all a simple matter. Reaching firm conclusions about their causes is likewise very difficult, not because of political correctness but (most likely) because it is very difficult to separate what is nature and what is nurture when you do not have a perfect laboratory in which to study such questions.
Whether such differences are truly as significant as many believe them to be, and whether these differences are genetic or a product of socialization, are among the most controversial subjects of debate in feminist studies and among others who seek to study sex/gender differences in a scientific manner.