What's wrong with this passage:
I kept holding it and said, "It's hard, but soft on the outside." Billy said, "Yeah, that's the way they are," with a trembling in his voice.
I'll tell you. It is one paragraph. I took this from an actual story, and this is only a small part of a 15 lines long paragraph.
Even this short excerpt should be two paragraphs.
Why?
Because in dialogue each character gets his/her own paragraph as they toss the conversation back and forth.
I kept holding it and said, "It's hard, but soft on the outside."
Billy said, "Yeah, that's the way they are," with a trembling in his voice.
That way it is clear who is saying what.
Also, a paragraph without dialogue is not made up of a random amount of lines or sentences. The simplest structure is to establish a subject for that paragraph in the first sentence, and then elaborate on it with a few more sentences.
Finished with that subject? Start a new paragraph.
A single paragraph that rambles on through more than about ten lines makes my eyes cross and my head hurt and I stop reading right there. That particularly applies when a paragraph includes dialogue.
You want people to read your story? Keep the paragraphs as short as is reasonable.
The rules for dialogue provide a great framework.
'nuff said?