Time is Short
An hour is short, like really short. It can fly by in the blink of an eye without realizing it. Before you know it, your day is gone. How many hours was that?
I don't mean in the sense of pulling out your phone, and the next thing you know, three hours have gone by. Nor do I mean it in the sense that you're swamped with meetings, and the workday is gone before you had the chance to do anything. Those happen too, but the issue is deeper than that.
Even if there are no external distractions and you're paying full attention to the present moment, an hour still feels short, like when you're engrossed in a game or in deep focus trying to write a blog post. But that's not it either.
What about when you're in pain? Surely an hour feels long when you're suffering? You could be in crutches or doing rehab after surgery, and it certainly feels like time is moving slowly, but that's only in the short term. After spending a few thousand hours, it would become a daily routine, and you'd get used to it. An hour would pass just as fast again.
Why do we think an hour is long anyway? Because it's long relative to a second or a minute? Or is it because it used to feel longer when we were younger? Since there are only 24 hours a day, does that mean a day is also short? Yeah, it certainly feels that way to me. That's why it's so precious.