Having watched a good number of Elon Musk interviews, I wish more interviewers would ask more direct questions. Asking Elon general questions usually results in fairly similar responses to what you have heard before or read in his biography.
But the answer might be shorter.
He has always said the chances of SpaceX succeeding was very slim - but at what point(after any particular technical achievement) he realized the this might work out.
I am pretty sure there have multiple of this yahoo moments successively - that made him believe that SpaceX will be able to succeed.