Anyone who falls in love is searching for the missing pieces of themselves. So anyone who's in love gets sad when they think of their lover. It's like stepping back inside a room you have fond memories of, one you haven't seen in a long time.
Whether you take the doughnut hole as a blank space or as an entity unto itself is a purely metaphysical question and does not affect the taste of the doughnut one bit.
Me, I've seen 45 years, and I've only figured out one thing. That's this: if a person would just make the effort, there's something to be learned from everything. From even the most ordinary, commonplace things, there's always something you can learn. I read somewhere that they said there's even different philosophies in razors. Fact is, if it weren't for that, nobody'd survive.