Seneca thought a close friend should be someone you can talk to almost as freely as you talk to yourself. Once you've chosen a friend with care, he wrote, "welcome him with all your heart and soul. Speak as boldly with him as with yourself" (Letter 3.2, Richard Gummere's translation).

But trusting someone with your worries doesn't mean they need every detail of a fight, the exact size of your bonus or a secret someone else told you. Some things tend to cost more peace than they're worth, even with people you trust completely. Here are eight worth thinking twice about.

1. Every detail of a fight with your partner

The Book of Proverbs puts the risk plainly: "He that covereth a transgression seeketh love; but he that repeateth a matter separateth very friends" (Proverbs 17:9, King James Version).

When you're upset with a partner, it's natural to want a friend on your side. The trouble is that friends often remember the worst version of the story long after you've made up. You forgive the comment at dinner. Your friend is still bristling about it at your birthday party a year later.

It's fine to talk about a hard patch. Just say what you need: "We had a rough week. I'm not looking for a verdict on him. I just need to talk it through." Then, when things improve, tell your friend that too.

That's about ordinary arguments. If you feel scared, controlled or unsafe, tell someone you trust what's happening. You don't have to tidy up the story first.

2. Every detail of your latest windfall

Xenophon records Socrates defining envy in a way that still surprises people. It isn't pain at an enemy's good luck, he said: "envy is felt by those alone who are annoyed at the successes of their friends" (Memorabilia 3.9.8, H. G. Dakyns's translation).

Modern research suggests he was onto something, at least in part. In a set of 1988 experiments, people's reactions to a friend doing better than them on a task depended on how much that task mattered to them. When it was something they cared about, a friend outperforming them seemed more unsettling than a stranger doing so. When it mattered less, a friend's success was a source of pleasure (Journal of Personality and Social Psychology, 1988).

Those experiments looked at performance on tasks, not at how friends react to news about money. Still, the practical point holds. Sharing good news doesn't mean sharing every figure. You can share that you got a bonus without sharing the number.

3. Secrets that aren't yours to tell

The Book of Sirach has a line that has aged well: "If thou hast heard a word, let it die with thee; and be bold, it will not burst thee" (Sirach 19:10, King James Version).

Passing on someone else's secret, even to a close friend you trust, creates a problem you then have to manage. You have to remember who knows what. You wonder whether it will travel further. And if it does, the person who confided in you will know where it came from.

When you're tempted, a simple line helps: "I can't share that one. It's not mine to tell." Most good friends respect it, and it tells them their own confidences are safe with you.

Keeping a confidence has limits, though. If what you've been told suggests someone may be harmed, staying quiet isn't loyalty. Tell someone who can help.

4. The same complaint, over and over

Seneca was advising his friend Serenus on whom to spend time with. "Above all," he wrote, "avoid dismal men who grumble at whatever happens, and find something to complain of in everything." Even if such a friend stays loyal, "one's peace of mind is destroyed by a comrade whose mind is soured and who meets every incident with a groan" (Of Peace of Mind 7, Aubrey Stewart's translation).

It's worth turning that advice around. Most people don't set out to be the friend who complains about everything. It happens one story at a time: the same difficult boss, the same unreliable sibling, the same grievance told for the fifth time.

Venting once can help. Repeating it can keep you stuck in it. If you notice you're telling the same story again, try changing the question: "Okay, I've said all this before. What do I actually want to do about it?"

5. Unkind things others said about them

"The words of a talebearer are as wounds," says Proverbs, "and they go down into the innermost parts of the belly" (Proverbs 18:8, King James Version).

"Did you hear what Sarah said about you?" can feel like loyalty. Often it just passes the wound along. Your friend now has a hurtful comment to carry, and you've become part of a conflict that wasn't yours.

The useful question is whether your friend needs to know. If someone is undermining them at work or spreading something false, tell them plainly and once, without the commentary. If it was just a careless remark, it can usually stop with you.

6. "I told you so" when things go wrong

Plutarch noticed that some people find the courage to criticize a friend only once that friend has fallen. They "assail him and trample upon him," he wrote, letting loose "a flood of frank speech." But when someone is already suffering, "there is no use for a friend's frankness or for words charged with grave and stinging reproof" (How to Tell a Flatterer from a Friend 28, Frank Cole Babbitt's translation).

His point was that the setback itself usually teaches the lesson. If you warned a friend about a business partner and the deal has just collapsed, they already know you were right.

What they need now is help. "That's awful. What do you need this week?" does more for the friendship, and for your own peace, than winning the argument.

7. A yes you don't mean

Sirach also warns about a quieter way of damaging a friendship: "There is that for bashfulness promiseth to his friend, and maketh him his enemy for nothing" (Sirach 20:23).

A promise made out of awkwardness often turns into resentment. You agree to help with the move, dread it for three weeks and then either cancel at the last minute or turn up grumpy. Either way, the friendship takes a hit it didn't need to.

An honest, smaller offer usually works better: "I can't do the whole move, but I can bring dinner over that first night." Your friend knows what to expect, and you won't spend weeks dreading it.

8. Unasked-for verdicts on their life

Plutarch believed real friends speak frankly, but he cared a lot about timing. He suggested watching for openings the friend gives you: "sometimes a question, the telling of a story, blame or commendation of like things in other people, may serve as an opening for frank speech" (How to Tell a Flatterer from a Friend 30).

An unprompted "Honestly, I don't know what you see in him" or "You're making a big mistake with that job" tends to make people defensive. It can also leave you feeling responsible for decisions that were never yours.

If something really worries you, you don't have to wait in silence. You can create the opening by asking permission first: "Can I tell you what's been worrying me about the new job?" If they say yes, say it once, honestly and kindly, then let them decide. If they say no, that's their call too.

Protecting your peace isn't the same as keeping things in

None of this means keeping your friends at arm's length. Secrets can weigh on the person who keeps them. In a 2017 set of studies, how often people's minds wandered to their secrets was linked to lower well-being (Journal of Personality and Social Psychology, 2017). Sharing good things can help too. In a 2004 set of studies, people who told others about positive events reported more positive feelings, beyond the effect of the good news itself (Journal of Personality and Social Psychology, 2004).

Some things should always be said, whatever they cost in the moment. If you're struggling, a close friend is exactly who to tell. If you believe a friend is in danger or being mistreated, speaking up matters more than keeping the peace.

The aim is to be thoughtful about what you share, not to stop sharing.

One place to start

Before you pass something on this week, ask yourself two questions: "Is this mine to tell?" and "Will saying it help anyone?"

If the answer to either is no, try letting it rest for a day. Most things that feel urgent to share feel a lot less urgent tomorrow.