1. Typographers and style manuals tell you to.
2. Two spaces were necessary when using monospace fonts; but now that everyone uses proportional fonts, it's no longer necessary.
3. It's ugly.
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I don't find these arguments very persuasive. I have my own aesthetic sense, and I don't need some authority to tell me what's ugly or not. I can tell that for myself, thank you very much. To me, two spaces between sentences look just fine.
Second, I find it easier to tell where one sentence ends and another sentence begins when they're separated by two spaces. A single space and a little tiny period is just not enough.. especially when reading at high speed, and when there are really long sentences.
If only one space separates sentences, and I want to skip to the next sentence, I'll have to read a lot more carefully than if there are two spaces -- when it becomes much more obvious where the separation is. Using one space in between sentences makes paragraphs look like one long run-on sentence.