Sentences are usually about one thing-- there's some unity there. But of course, the reason we have more than one sentence in most paragraphs (and we do, don't we???) is because we want to elaborate or define or refine beyond the first sentence's boundaries.
Something I often edit for is sentences that aren't unified. I can't exactly explain, alas, but let me give you an example:
Mom might be saying "jump." Trudy didn't have to ask how high, and she was too old to be ordered around.
Two sentences, and it's all related to Mom ordering Trudy around. But notice that the first two elements are riffing off that old saying, "If I say jump, you ask how high?" Those two really belong together. Putting the second element in with the third is going to lose that unity and the echo of that saying. So try it that way (still need a conjunction for the second clause in a sentence):
Mom might be saying "jump," but Trudy didn't have to ask how high. She was too old to be ordered around.
Now you can see that the first two clauses go together, and the third is in a sentence of its own because it's a conclusion to the combination of the first two.
Actually, if you read that aloud, you'd hear that the first two are meant to be together. Often the rhythm will tell you that something is missing or something needs to be conjoined or separated. Listen. Your inner ear and your mind are connected. :)