Why is it so easy to spot typos in things other people have written, but so hard to find them in your own writing?

Answer 1

What I learned in my English courses in college, if you are proofreading your work, read it backwards, sentence by sentence. That way you don’t lose concentration on looking for typos by being distracted with your ideas within your written piece, but you are reading sentence by sentence independently and you will be able to spot typos better.

Answer 2

You know what you’re (trying) to say, so when you’re reading your work it’s easy to gloss over things. It’s best to take a break and read or do something else, and then come back to your work to reread, review, revise, edit, proofread.