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

[Pervinca Took]

In many cases, health follows will: a good outlook and positive emotions can hold physical pain at bay, or at least lessen it. Frodo did not have the benefit of that himself.

Perhaps he had it, at least to some degree, once he had actually made the decision to sail. That was a positive decision to do something about his suffering, to take the best path open to him. Tolkien said that it was after his last grievous illness - in March, I think? - that his mind was made up. Also, he left before the October illness could recur. That is even spoken in an earlier draft: "I'm going now, before the wound returns."

Tolkien does speak of some positivity - "if Sam thought he was lucky, Frodo knew that he was more lucky himself ... he took to a quiet life, writing a good [or was it great?] deal, and going through his notes." I think he used the near-completion of the Red Book as a focus. Sam did remark "Well, you've kept at it, I must say."

There was also, reputedly, much coming and going between Crickhollow and Bag End. I always took that to mean that Frodo visited Crickhollow as well as Merry and Pippin coming to Bag End, but I might be wrong.