An interesting question.
I would risk to venture these thoughts: Leica M _*users* (as opposed to mere purchasers) share a number of traits which include a passionate love of photogrpahy as a narrative art for (as distinct from the purely aesthetic), an intense interest in the human condition (both in its more extreme and quotident iterations), a fetishistic love of beautifully conceived and executed machines coupled, ironically, with goal of having as little technological mediation between their eye and the subject, a dichotomy which also takes shape in a desire to be unburdened by their camera equipment while milly being obsessed about realizing maximum image quality.
Leica users also tend to be either sufficiently dedicated to these goals to make the sacrifices necessary to afford their tools of choice, or are successful enough in their endeavours to do so.
- N.