Not giving this driver a good tip

Was it all a dream? Was he mentally insane or just really stupid? What about the character was the director trying to portray to us. What was going on in terms of character development? Most of the movies we watched I had some form of an answer to these questions, but when it comes to Taxi Driver with Robert De Niro, I don't have a f*cking clue...? Not only can I not even begin to think of an answer I don't even want to think about this movie much more than I already have; too many annoying migraines come about the more I get into it. Maybe that's what the director was trying to do? Make the audience leave the theater with so little to think about that we end up creating our own ideas and over thinking everything. The one thing I know about myself is that I like some form of certainty. Without it I cannot think straight. Movies like The Shining left us with some sort of starting point for why Jack went insane; the ancient hotel built on a native burial ground that's infested with ghosts gave me a good starting point. I believe Taxi Driver fails to give us any deeper in sight into the psyche of Robert De Niro's character; and how he falls into his twisted dark vigilante role. This dramatic change of character happens pretty early on in the movie, only giving us a good 20-30 minutes to really get to know him before the psychotic episode begins. I tried extremely hard to understand exactly what it was that set him off initially. It could have been that his lady friend rejected him, but I don't believe that something like that would have turned him into a potential presidential assassinator. What I do know is that I will never fully understand what took place over the several months where Robert De Niro went on a violent mental breakdown. I also know that his acting was the only thing worth recognizing throughout the whole movie. I mean it is Robert De Niro after all...