Some folks may consider a momentary opening to be "shallow" or "deep". And, yes it is true, such openings may come in tastes "big" "small" "long" "short" and beyond all consideration of size and time at all.
However, all such passing glimpses are "shallow". All shallow, by necessity. The "depth" comes from the day to day slog of life: putting stamps on envelopes, gas in the car, a bandage on a child's skinned knee. People do not know what "enlightenment" is, thinking it some momentary bit of fireworks or unusual sensations outside our normal perceptions. A glimpse of interconnection and stillness is worthless without a return to our world of disconnection and chaos ... though, hopefully, we can learn to taste some of the stillness in the storm.
People practice Zen for the fireworks. It is a shame, however, if they do not know where to find fireworks in the mundane, ordinary, just plain tedious disagreeable dangerous and dull. A passing opening or dropping away is no more than a visit to the wide, empty spaces of the Grand Canyon ... nice place to visit, but we can't and should not live there. The real learning is not there. Yes, we may wonder at the majesty and beauty, take a breath beyond space and time ... but we should not (and cannot) stay there forever while in this life. We must very soon get back in the car, get gas and head back into ordinary life. We have to be at the office on Monday, or there'll be hell to pay.
Not that we do not value such experiences, but value walking the dog or making toast as much.
There is no self. But the dog needs to be walked.