II don't know since when I've been cynical but I suspect it may have something to do with kids in my class in primary school insisting that "the sun is obviously a planet, what are you talking about?" Seeing people getting away with a lot, later on, reinforced that.
With time, however, I found that cynicism, although easier to adopt, made me question the point of everything and most importantly, was making me miserable. Staying somewhat idealistic, on the other hand, made me happy, not for the [disappointing] others but for myself.
In short, nowadays I dwell on the fine line between cynicism and idealism.