The natural progression of life means that everything inevitably changes. The trees, the animals, the taste of the air, change. The tides, the moon and even the sun change in small degrees.
That's why Donghae knows he shouldn't feel guilty or sad that people around him have changed, or that relationships he held no longer exist.
So the text message he gets from Kibum months after he's seen him is a bit surprising, to say the least.