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?
To me, it just means that you need to be mindful of it being a partnership. If they’re good at giving emotional support, while you’re good at handiwork or whatever, and you’re both happy with that arrangement, then there’s no point in mulling it.
It’s more a word of warning, especially for the traditionally-male raised folks, who might come into a relationship and fall apart completely, because they feel for the first time, like they’re allowed to share their hardships. Like, yeah, you can do that thing, if you’re mindful of them being on board. You do still need to make it up to them, because it is a partnership. And you simply won’t know, if they can even emotionally deal with your troubles. There is always the risk, especially in young relationships, for your partner to need to get out of there, no matter how much you’d be willing to make it up to them, because it simply drags them down far beyond any relationship could make up.