You start to use them for emphasis, which slowly leads you to rely on them for establishing any kind of contrast, which devolves into a rather lame mimicry of your intonation. Eventually, each sentence becomes overpopulated with emphasis, rendering the text cheap and slightly condescending. The reader (usually) isn't stupid, so they should be able to work out what's surprising or contrastive on their own. If they can't, or you feel that italics is the only path to their understanding, then maybe it should be put differently in the first place. Try replacing all your italicised emphasis with bold face, and see how excessive it looks.

# Footnotes

This opinion is completely devoid of any authority, and is expressed here only as a minor grievance. Really, I just think italics ought to be reserved for marking structure on the page, such as in setting a running head in italics to distinguish it formally from the body text. But I acknowledge I have a more renaissance view, and prefer not to mix roman and italic faces on the same line if it is avoidable. Small capitals or bold to me are better capturers of emphasis.