Essayist Verlyn Klinkenborg has a more erudite and complicated answer about the origins of sentences in this New York Times "Draft" column:
Now try turning a thought into a sentence. This is harder than it seems because first you have to find a thought. They may seem scarce because nothing in your education has suggested that your thoughts are worth paying attention to. Again and again I see in students, no matter how sophisticated they are, a fear of the dark, cavernous place called the mind. They turn to it as though it were a mailbox. They take a quick peek, find it empty and walk away.