Most conventions in English have exceptions. One such convention is a phrase you’ve probably heard in some English class, whether it was in middle school or college. “Write the way you speak.” When we’re talking, we never consciously decide, “Oh, I’ll put a period here. This is where the paragraph of this speech ends. I need a comma here.” But in writing, we do need to consciously decide where to put punctuation marks.
Because the reader is reading your words on a page, and not listening to them, they don’t know where you’re pausing or taking a breath. The periods and commas stand in place of these breaks and help them navigate your words. You might have a long sentence that you believe is necessary to keep together because the ideas all depend on each other. However, when writing, it helps to break up those ideas a little more and use transition words to show the relationship between the now separate sentences.
“I believe this candidate is the best for this position because she likes chocolate. (Furthermore, Also, or Moreover), she helps those in need by listening to them and cares for animals in her free time.”
You could combine all of those three traits into one sentence, but it might be a lot for your reader to keep up with. If your reader is your professor, you definitely want to make their job as easy as possible, because then you’ll get a better grade for helping them through their reading experience.
Although long sentences have a place in your writing, when sentences get too long, your reader can get lost. We have the ability to make infinitely long sentences with more and more clauses. But in both speaking and writing, we usually don’t. The purpose of writing is to communicate ideas, and short sentences seem to help us communicate better. So when you’re writing, try to distance yourself from your writing. Is this sentence long? Is there a way someone could get lost or confused? If you don’t want your reader to need to put in a lot of effort to understand your point, it’s best to just break it up just in case.