This phrase can express enthusiasm or obsession, showing strong personal interest. He goes on about his trip to Spain, lighting up with every detail.
Pro Tip 2/3
Conversation Queue
Used when someone doesn't give others a chance to speak, dominating the conversation. During the meeting, Helen kept on about her project and nobody else could contribute.
Pro Tip 3/3
Listener's Interest
Be sensitive to listeners' interest; they may find the topic tedious if overdone. Mark is going on about cryptocurrency again, and I can see people tuning out.