The reason we do it is because its a natural human factor. Its not so easy to change a plan once you're doing it, there's a level of annoyance if your'e compelled to.
For example your driving your car and come to a detour and complled to drive around side streets only to emerge 100 metres down the road, we do it because we are compelled to but there's a level of annoyance. That level of annoyance is dependant on how much inconvenience is involved.
Take the classic example, you are more than half way through painting the loungeroom wall when the other half comes in and says, oh no that isn't right, it needs to be darker. Your reaction isn't no problem luv, I'll just add some darker tint and start again. is it.
The reaction to a plan change once executed is dependant on the level of inconvenience and whether or not you are compelled to.