"They all say life goes on. But no one ever says that it's a good thing."
"...why is that?"
Here's one overly romanticized version of an answer I might have.
Sure the world will not stop revolving around the sun,
the seasons will prepare for the next cycle,
while the next daylight would not cease to diminish.
Hence the body tries to go forward,
the mind tries to grab whatever makes most sense...
while the heart... stops on its track...
Moving forward is not difficult.
Bearing with the pain of moving forward with the heart left behind...
is.