[End of paragraph] . . . Helena said goodbye to Martin.
[Next Paragraph] Martin ate his now-cold lunch.
Where's the connection? The second paragraph could begin with: Four hours later, Martin ate . . . Or you might add a scene break. In typewriter days, we wrote [Double Return] to show this. Today, it's as simple as 2 centered asterisks, or § (alt 0167 on your number pad), or [page break].
Here's the rule I follow in fiction: As soon as I finish one scene, I lead readers into the next with no excess prose between them. The details of how the characters got there can clog the flow of the story.
I write transitions to make reading smoother.