"All intelligent people should think primarily in terms of opportunity cost. When deciding whether to do something compare it with the best opportunity you have."
This quote is from Charlie Munger and resonates with one of the decision heuristics I use the most: When you are faced with a choice between two plans, choose the one that makes the most sense given the timing. Something you can only do during a certain time window, under certain circumstances, at a certain age.
You can only explain "Why now" well when you have considered your opportunity cost seriously.