Dwell on

To dwell on something is to focus on it obsessively. This could be in thought, but also in words, either written or spoken. If you dwell on something, it means that you keep thinking, writing, or talking about it for a long time. Usually, the feeling of this phrasal verb is not entirely positive, although I suppose that many problems are solved because people dwell on them! Maybe the sense is negative because other people are sometimes bored by repetition, so if you dwell on something long enough, they'll become irritated. Here's an example conversation:

Him: "That happened years ago! Why do you keep dwelling on it?"

Her: "Well, I guess I'm still upset about it!"
Him: "I don't see that dwelling on it is going to make you feel any better!"

The image here is Automat, by Edward Hopper.