Treat entrusted information as belonging to its owner
When someone shares a personal concern with you, they have chosen one audience. Retelling it to another friend changes that audience, even if you mean well. Checking before passing it on respects the difference between being trusted with information and being authorised to distribute it.
Go deeper
Being told is not the same as being asked to tell
A friend describes a difficult conversation at work. Later, another person asks how the friend is doing. It can feel natural to repeat the story as an explanation.
But the original disclosure had a particular audience. The person may not want the detail circulating more widely, even among people who care about them.
For ordinary personal information, asking before sharing keeps that choice with its owner. If you need general input, you may be able to discuss your own uncertainty without including identifying or unnecessary details.
The principle is not about making every conversation secret. It is about avoiding the assumption that closeness to someone grants an open permission to retell their private experiences.