What were the initial motivations of the use of the proper forcing.?



The history of Proper Forcing
I agree with Andres that this is a very ambitious question. Let me throw in a tiny little bit of information: 


As far as I know, Jensen's construction of a model of CH without Souslin trees was one of the first uses of both countable support iteration and master conditions (conditions generic over
a countable elementary submodel of a sufficiently large initial part of the universe).


One of the first publications using proper forcing seems to be Shelah 100, with the hilarious title "Independence results" (JSL 45 (1980), 563-573).  

This paper not only introduces proper forcing (without proofs of the the iteration theorem and so on), but also oracle-ccc forcing and oracle-proper forcing.


