以下是为女儿回答她同学的问题，由于是非华人，所以用的是英文。
要注意，这只属于轻松的，非严肃的解答。从历史的角度，有些细节可能并不严谨，（如要严谨，篇幅就太长了），有些仅仅是我自己的观点。
不过对有同样疑问的，这些回答，大致会有帮助的。
Why did the Elves have to leave the Middle Earth for the Undying Lands?
No, none of them had to, they went as they would.
In Tolkien’s earth, Arda, there are two major continents. One is the Undying Lands (a.k.a. Aman, or the Blessed Realm), where gods live; one is Middle-earth, where the children of the creator (Eru Ilúvatar) were awoken. The elder children were Elves, and because they were immortal, they suffered from bad things (evil gods, orcs, etc.) forever; the younger children were Men, who were more beloved by the creator, so he gave them a special gift – death, so that they wouldn’t have to experience eternal suffering. Out of pity for these immortal Elves, the gods called for them to leave Middle-earth for the Undying to live without suffering.
During the First, Second, and Third Ages, many Elves realized that Middle-earth was not really their home because they were tired of terrible things. Some, like Nordor, elves of Galadriel and her father's race accepted; some, like Sindar (Grey Elves), elves of Legolas and his father's race accepted but delayed leaving; some went and then came back to Middle-earth like Galadriel, for their own reasons. Most who did leave left from the Grey Havens, which is the harbour we see in the last scene of the 3rd movie.
After the War of the Ring, most Elves thought the time of the Elves was over and Middle-earth was for Men, so they (including Galadriel, Legolas, Thranduil, Elrond, etc.) finally decided to leave Middle-earth. Again, none of them “had to”, but they went as they would. Some stayed until now. People say we are now in the Sixth Age and they’ve evolved into phantoms, and that’s why nowadays people see ghosts,LOL!
How did the Elves know that the time of the Elves was over?
You remember in “The Fellowship of the Ring”, Elrond said to Gandalf, “The time of the Elves is over, my people are leaving these shores”? The simple answer is: Elrond said so, haha!
The detailed answer:
At that time, there wasn’t a concept of democracy, so people lived under monarchies. For those who had left for the Undying Lands and come back to Middle-earth (like Galadriel), they were OK with living in Middle-earth when their kings were still around, but after their last king Gil-Galad died (there was a brief scene of him in the first movie), they gradually gave up and sailed from the Grey Havens to live under the other monarch, King Finarfin (Galadriel’s father), who lived in the Undying Lands. For those who had accepted the invitation to leave but delayed it like Legolas and his father (the Grey Elves), their king had died a long time ago, so they left part by part for the Undying Lands to live under their king’s brother, another king.
So in the time of the LOTR movies, there weren’t many Grey Elves left. Most elves you saw were Wood Elves (including Evangeline Lilly’s Tauriel). They were of the same race as the Grey Elves, but separated long ago (like Finns and Hungarians, LOL!). Their king had died long ago, so they were loyal to the Grey Elves’ king, but not directly under him. They were scattered somehow. Some part of them took Legolas’ grandfather as their king and formed a kingdom. He died later, so Thranduil took over. So you see, Thranduil formerly was of no Elvish royal blood. He also “needed” to leave to seek his monarchy. So the Wood Elves also eventually needed to go.
The ones who chose to linger for a while, like Galadriel and Elrond, did so because they thought Sauron had not been completely defeated yet. After the War of the Ring, they finally thought it was over, they should leave, so almost all Elves decided to follow.
What about the Orcs, Hobbits, and Dwarves?
The Orcs had been defeated completely. The Dwarves might have been like the Neanderthals, haha！eliminated from history.
As for the Hobbits, their population was so small, so they probably also passed out from history. For thousands of years, Elves and Rangers (Aragorn’s race) had an unsaid mutual agreement to jointly protect the Shire – the Hobbits didn’t even know! When this protection was gone, they probably also failed to survive.
