People do weird things under pressure.

Yep, which is why I think it is good to have some challenges to face,, to see how someone reacts. It is hard to predict in the good times.

A happy relationship is a place where there is a lot of elasticity.

I think this requires skills on both sides, as well as a willingness to find compromise and put some personal things on the backburner sometimes. Not all the time though.

And when children come into the couple's life... They whole dynamic changes.

Yep. children add unknowns on many levels. It is impossible to prepare for everything specifically, so having some process to be able to cope generally is important.