If a paragraph is too long, it’s hard on the reader’s eyes. Break them up for visual appeal, even if in a technical sense it’s only one thought. I don’t know about you, but when I come to a paragraph that takes up an entire page, there’s something daunting about it. Sometimes they’re even hard to read, depending on the font size and spacing.
There are so many ways to distract a reader it’s pathetic. Long, run-on paragraphs are one of them. Even if it’s the same thought, be sure to break it up if it takes up half or more of the page. If it happens to be dialog, you may want to break it up with some action. Otherwise, remember that when the same speaker continues to the next paragraph, you don’t use a close quote on the previous one.