Firstly, no idea about the origin of this phrase, but it seems like a poorly constructed idea that is broad enough to generate discussions on any point of view that you want.
What I take from researching is that sharing your troubles shouldn’t be a one way street, if both people support each other it isn’t therapy it’s sharing, which is just semantics at this point. Can I vent about life to my partner or only to my close friends? or do I need to hire a professional?
Alternatively a very direct reading is that your partner doesn’t have the medical expertise to solve your mental issues. Which is very fair, but again lay people can’t be expected to diagnose if their partner is suffering from depression or just sad that their dog died.
A really negative reading of this is just a toxic excuse, “Oh, my boyfriend cried in front of me so I dumped him, I’m not his therapist”
At the end of the day, ignoring this phrase, should I share that I had a bad day with my partner? Should we talk about trauma that we had as kids? Where is the line if there is any?
Sounds like another way toxic people continue to divide us in our own relationships. My wife and I support each other emotionally during hard times. If it’s both of us, or one of us, our job is to be the support the other one needs so they have an easier time coping with it.
Your partner is your best friend. They’re not your therapist, they’re your other half. If you can’t be open, and honest with them - then what’s the point?
We even have laws which say Husbands/Wives can’t be forced to testify against one another because that bond is so strong that they share things that can’t be shared with anyone else.
As with anything though, you just openly discuss where that line is with your partner. Communication is key in any healthy relationship, and the moment you stop communicating or hiding things from one another - it just slowly slides downhill from there.
If they use that vulnerability as ammo against you - call them on it. Your SO is effectively, supposed to be the person you can trust more than your own self at times. If they aren’t that for you, you gotta decide where that sits in your heart.