Why is it

Why is it that you can be having an ok day, maybe even a good day, and better still a great day, and then one small thing, one small event, something you see or hear which triggers a momentary thought that spreads through you thoughts and feeling overwhelming everything else where you just want to collapse right there, even want to hide from the world in the darkness of a room, and the rest of your day is gone, absorbed completely by that one momentary thought which just through happenstance happened when the right events happened in the right situation under the right circumstances to destroy your whole sense of yourself, your whole being, you, gone in a momentary lapse of reason?