Why are these groups often so great, so welcome, so precious?  Real life is very rich - theories only capture aspects of this richness.  However a theory, that I like a lot, highlights one reason why these peer groups are so important.  The theory is Self-Determination Theory (SDT).  It has evolved for over three decades.  The SDT website (see below) is a treasure trove of information about this approach.  It contains hundreds of research papers covering SDT's application to many fields including happiness, wellbeing, friendship, couples, parenting, education, psychotherapy, healthcare, political/ecological action - to name just some of the more obviously relevant. 

SDT focuses particularly on the crucial importance of satisfying three basic psychological needs - autonomy, competence and relatedness.  It proposes that:

1. A basic need (whether physiological or psychological) is defined  as an energizing state that, if satisfied, promotes health & well-being but, if not satisfied, contributes to pathology and ill-being. 
2. How these three basic psychological needs can best be satisfied will vary with individual strengths, life stage, social context, and culture. 
3. Personal goals that lead to satisfaction of these three basic needs will promote well-being, but individuals - due to broad societal conditioning and personal life history - may well strive for goals that do not satisfy these needs or enhance their well-being.

SDT has found that individuals who satisfy these three needs better (for autonomy, competence and relatedness) tend to have higher wellbeing than those who satisfy these needs less well.  Similarly for any given individual, days when these three needs are better satisfied tend to be days that feel better than days when the needs are less well satisfied.  SDT also finds that it is important to satisfy all three needs.  Just because, for example, one is highly autonomous and competent, it doesn't mean that one can ignore the crucial importance of relatedness (and similarly with any other of the three needs).

Looking more deeply at ‘relatedness' (feeling close and connected with others), researchers have explored what kinds of social activities are particularly beneficial (Reis, Sheldon et al. 2000).  They asked subjects to keep diary records of their social interactions for a couple of weeks and track seven different kinds of social activity.  They found that - for both men and women - it is interactions involving "talking about something meaningful" and "feeling understood and appreciated by others" that are most closely linked to ‘relatedness' needs being met.  This is turn links strongly with increased positive mood and wellbeing.  Interestingly "participating in enjoyable social activities" is also associated with increased wellbeing, but not apparently via increased ‘relatedness'.

So I suspect that one reason why these peer groups are so precious is that they are deeply concentrated opportunities for satisfying SDT's relatedness need to feel close and connected to others.  They involve repeated experiences of "talking about something meaningful" and "feeling understood and appreciated by others".  This, in turn, leads to increased feelings of positive emotion and wellbeing ... and hopefully too, an increased appreciation and ability to nourish feelings of relatedness in one's life outside these groups.

The Self-Determination Theory website is at www.psych.rochester.edu/SDT .  Accessed on May 12, 2008.
Four downloadable Powerpoint slides on SDT produced by James as a two slides to a page handout.  [1st Page; 2nd Page] 
Reis, H. T., K. M. Sheldon, et al. (2000). "Daily well-being: the role of autonomy, competence, and relatedness." Pers Soc Psychol Bull 26(4): 419-435.  [Abstract/Full Text] [Free Full Text]