You should keep sentences short for the same reason you keep paragraphs short: they're easier to read and understand.
Each sentence should have one simple thought. More than that creates complexity and invites confusion.
Think about "The Hobbit" Not going after Tolkien but how many of us got confused in the hobbit and how long did it take you to read? Personally I love the hobbit but I think we all had a part where we dozed off a bit.

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How to make your writing better without sucking
RandomWhenever I write any story I always get writers block or something like that, as it says I am not a human perfectionist but I try to make the most out of something I do, my making this book I will help myself and others who feel the same and possibl...