They handle their own bad days

They share what they're going through — they don't perform stoic silence or pretend everything is fine when it isn't. But they don't dump. They don't arrive home and convert their mood into the atmosphere of the house. They don't make their partner responsible for managing an emotional state that the partner didn't cause. They share, ask for what they need if they know what it is, and do the basic work of regulating themselves. The relationship is a place for support, not for outsourcing your nervous system. Partners who consistently use the relationship to manage their emotional states — who need the other person to be emotionally available on demand, to absorb whatever mood has arrived — create an exhausting dynamic that slowly depletes both people. The support a healthy relationship offers is finite and needs to be treated as such. You can lean. You cannot transfer the weight entirely.