(Not particularly rationalist here, but.] In my case, it was a series of unpleasant things over which I had no control. When we were about 6, our parents had to send my sister and I to live with grandparents (due to financial uncertainty.) We mostly liked it there, though gr.-p. often had loud quarrels (to this day I hate shouting most of all. It helped, when puberty ended, to argue calmly.) Yet I had a recurrent thought - what will happen when our grandparents die? And I was revolted with myself so much I flat out decided not to think it; but later, when we were shipped back to our parents and it became safe to think again, I returned to it. I know what a resolution to omit something in my head feels like.

We returned to our parents at the adult age of 12, into a different country with a different language (Ukrainian) taught in schools, a different moral in history lessons, different everything. We had each other, and a raging case of Us vs. Them. It took years to admit to myself that maybe I don't have to completely break the way I think to fit in, mostly because we changed schools and were blessed with having an extraordinary biology teacher. (It was then that I learned the value of belonging to a group with common interests, and it shaped my life choices afterwards.) Our father is a physicist, and very pedantic in speech, which we tried to follow as a model. It didn't endear us to our classmates, which gradually showed us the weaknesses of being untimely pedantic. ...in college, I had a hothead friend who passed judgement on people quickly and forever. I vaguely understood he was wrong in this; and was at once assigned his 'conscience', for a short while. It made me willing to at least hear the other party out. ...I realize this sounds like a cliché, but being an atheist among Christians was useful, too. I had many opportunities to not let the argument be swayed into religious disagreements, or to leave when it was beyond me.