I think we tend to think that things (i.e., our hair follicles) are frozen in time when we shave -- and that if we stop shaving our hair will look exactly the same as did previously. But of course, those with MPB continue to lose hair. You don't notice it as it's happening and then you skip shaving for a few days and notice that nothing is growing back. It can be a bit of a shock. However, we should look at it as a confirmation that we made the right decision by shaving in the first place. Most people want to hang onto their hair for as long as possible , but I maintain that it's better to shave sooner rather than later (i.e., before you "have to") and spare yourself the anguish of watching your hair fall out strand by strand and seeing your hairline recede up your forehead.
When I cut a big chunk out of my head and had to rest shaving for a week I was shocked how much of my remaining hair was grey!