The following are some rules I've picked up about what makes physical
interactions socially acceptable.  These are all about platonic friendships;
couples have different rules and expectations.


Specific Rules:

It is more acceptable to initiate a particular physical interaction if...

* you do it with everyone [than if you do it with just one person]
* you see the recipient do it with lots of people
* you've done it before with the recipient
* it's similar to previously accepted physical interactions
* you [seem to] do it automatically, subconsciously, or unselfconsciously
* previous initiations of physical contact have been met with positive
  responses


Overarching Principles:

* Physical contact, particularly when negotiated non-verbally, is more
  likely to be accepted if you are (or seem to be)
  doing it for the other person's sake at least as much as you're
  doing it for your own sake.
* Acceptable physical contact is generally context dependent; "it's
  pleasurable" is only a good enough justification if you're dating, while
  "you're sad, and I thought a hug would comfort you" is usually acceptable.

Note that people can usually pick up on your justification/reasons.  If
physical contact needs to be verbally justified, any 'acceptable' justification
is more likely to be a rationalization, and not the true reason.