Why did Frodo feel he had to leave Middle Earth at the of ROTK?

[Lotrelf]

Originally Posted by Zig�0‹4r

The reason we don't consider Aman to be Marred like the rest of Arda is because unlike the rest of Arda the matter of Aman is not combined with Morgoth's spirit, something he did in Middle-earth.

Would you please elaborate explaining this?

So the Trees were destroyed, but they and the land at large were not actually corrupted.

Is it also because the Fruits of the Trees covered the entire Arda with their light- night and day, and even Morgoth feared them.?