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

[Lotrelf]

Originally Posted by Inziladun

Although Ungoliant did kill the Trees, the land of Aman itself was not harmed, corrupted, or affected by Melkor, as he had done in Middle-earth. Aman was the design of the Valar indeed "unmarred" from that viewpoint.

Does only the land matter? Trees were killed/destroyed. There was darkness all around, if I remember that correctly. It's like saying poisonous air is not in the land so those who live on the land do not need to fear it, while the birds etc. are in greater danger. It 's a bad analogy, but for now I can't think of something better!