Theory:

In every relationship there is a "worry quota" that must be met. Usually (in healthy relationships) the quota is met in shared fashion. Sure, one is usually the worrier and the other is the more care-free, but usually, in the face of real threat to security (finacial, health, etc.) both pitch in.

It's like a manifestation of our animal survival tendencies -- animals do that. If you ever "invade" a field of cows, invariably one cow will separate itself from the rest of the herd -- a protector. Pack animals act very much like a group of people that are threatened. There's no need for all in the group to lose sleep. One of the group is set out to be a watchman so all can rest easier. But it only takes one.

But it takes one that you can trust. And that trust can be broken by factors real or percieved.

A pessimist in the family damages the worry quota more than the optimist can heal it.