This depends on what you mean by "rationalist." If you mean having certain opinions, there will always be a backstory, since acquiring opinions is a historical process. If you mean being interested in rationality and biases and so on, there will always be a backstory, since people's interests develop over time.

If you mean the characteristics that make people want to be reasonable, I suspect that most of these are personality traits and in that way people will have "always thought that way." This is just as true of extreme personality traits as of non-extreme ones.