Quite often things which appear to be weak survive, whereas on the other hand things which appear to be strong fall apart. How come?

Example 1: 

A country which is threatened from all sides often survives, because the people are alert, and a common enemy makes them hold together. In a country which is strong, people tend to become lazy, and they tend to do what is best for themselves and not for the common interest.

Example 2:

Marriages which seem to be "impossible combinations" last, whereas dream-couples divorce. Maybe keeping up an impossible combination against all odds is a higher incentive than to maintain something which anybody anyway expects to succeed?