We are not omnipotent and omnipresent like God. We are not like Him who seems to have a zillion eyes and ears used to monitor our intentions and actions and act accordingly. (Or some may argue that maybe God is not that powerful, after all. Why, perhaps He's so limited that He cannot watch over all of us, given the many accidents that come our way.)
Humans as we are, we cannot do all things at the same time. And what we are doing now, at this point in time, is an opportunity cost for other things that can be done. Everything is a trade-off.
When one is reading a good book while riding a bus, she loses the chance of being awed by the majestic natural creations: farms, mountains, seas, and all.
When one works abroad, living a very good life and earning income more than enough for her family in her hometown, she loses the chance to serve better in her own country.
When one excels in work and is hungry for more success, she loses the chance to be with her family and be closer to them.
When I'm home and get immersed in writing with Isda, my technological soul mate, I lose the chance of playing with my kids, who definitely need care and concern from their parents.
So it becomes a question of values that inform our choices. They define the trade-offs. And it requires a skill in how to identify them, so that after all the decisions are done, there is less or no amount of guilt left in our hearts.