It is always the little things that build up. Often there is no dramatic reason for discontent in marriages. It seeps in slowly over the years.
You don't even notice it creeping in. It happens, trickle by trickle.
You do not realise when or how the easy familiarity gets replaced by a 'taken-for-granted' attitude over the years. By the time you do, it is often late. Habits have been formed, patterns have been set. And a comfort-zone have been established.
A zone that is hard to get out of.