This is a good question. I would say that he chose many things. First, he chose to move to Thebes. He chose to strike and kill a man on the road, who would turn out to be his father. More importantly, he undertook to find out what the problem was concerning the plague. He took this upon himself to be a "savior". His zeal was enormous, or we can even say his pride was enormous. By this point of the play, Oedipus is completely driving the story every step of the way. It is all about his passion to find a solution. He did not have to do these things, but he chose to. Perhaps this is what led to his downfall. Perhaps this is why he is so tragic.