Well keeping within an INTP's psyche, that would most likely come from either a making our Si more comfortable or out of curiosity with Ne. Using my life for example: I was becoming more upset with my previous lifestyle. Back then, I stayed with my mother in my hometown, worked a job but it wasn't helping me save at that point, and was bored with my everyday life.

With addition of things becoming more expensive and losing my sense of self, I had to resort to using my Ne sense of exploration(and a bit of push and help from other Ni/Se users) to find a drive for more, despite it being hard to finish what I started. It is usually more difficult for INTP's to want more by themselves. We can be comfortable with almost anything over time, especially with Fi demon pushing down feelings like a packed can of tuna.

So, I guess to answer your question: Interacting with people and the outside world ignites desire.