Why is the ending so sad in the Return of the King

[Joy]

You know, epics are not supposed to end. Remember when Sam and Frodo are in Mordor, speaking of the great tales?

Why, to think of it, we're in the same tale still! It's going on. Don't the great tales never end?"
No, they never end as tales, said Frodo. But the people in them come, and go when their part's ended. Our part will end later - or sooner."

The tale of the Fellowship and the War of the Ring may indeed be over, but the epic is not yet over. Their part is done, and were are here today, battling some of the same evils.

How will your part play out? Will you be loyal like Sam, or will you be a traitor like Saurman? Will you falter in your pride, yet redeem yourself like Boromir?