Italics are often used in writing, to put emphasis on a word, or even a phrase. They make the word stand out in our minds, and we understand that it is very important. It catches our eyes. Used sparingly, it's a good tool to have.
Italics look like this. They're nice for a while, but soon they get a little annoying, don't you agree? Like this. I could go on and on and write this entire post in italics. I probably should, just to make a point. But I won't, because you are my faithful blog readers and you're awesome because I now have thirty-six followers. Yay!
They can also be used in dialogue. For example, look at these examples:
"I hate you."
"I hate you."
"I hate you."
In the first sentence, the words are relatively equally spoken. No word is placed above another. The speaker doesn't emphasize any one of them. In the second example, the speaker places emphasis on the word hate. He/she hates you, and wants to make sure you understand that he/she doesn't merely dislike you. They hate you. In the final example, the emphasis is placed on you. The speaker hates you. Not Bob or your aunt or Lauren Lopez. You. If you listen to people speak in real life (real life...what real life? Oh the horror.), you'll notice that they don't say each word equally. They emphasize words. They draw them out and speak them differently.
Great. Now that I've shown you the wonders of italics, we can get to why I really wanted to write this post. To show you the horrors of italics. Go look at the second paragraph again, the one almost at the top. Do you see how annoying it is when I italicize too many words in one sentence? In one paragraph? Not only does it hurt my eyes, but it hurts my Inner Sense of Writerness, too. In other words, it makes me cringe.
Italics must be used sparingly. I repeat: sparingly. Only when you really, really need them. A good rule of thumb: if your writing still works without the italics, cut them. It gets annoying to the reader when they start finding italics on every other page.
Now, I've never seen something as dreadful as my example in a published book. I just did that to make a point. But I have read books where the author used excessive italics. And it was annoying.
If you use too many italics, they start to lose their meaning. Your reader no longer cares about the italicized word, because they've seen so many others that they've just stopped noticing. And you don't want that.
So, how often should you use italics, then? There's really no way of giving a definite answer. I could give you numbers, but they'd be meaningless. That being said, I wouldn't use italics any more often than every four pages or so, at a minimum. If your reader is starting to say "Wow, that author uses italics a lot.", then you've gone too far. If they don't notice, you might be fine.
Don't abuse italics, people. It doesn't make for good writing.