People assume this pairing is a mess waiting to happen, two dreamers with no map, both perpetually late and neither one willing to handle the practical stuff. From the outside it looks like a relationship held together by vibes alone. From the inside it feels more like two people who finally stop apologizing for how their minds work, because the other one gets it without needing an explanation.
Both signs are mutable, which means neither is trying to run the show. Sagittarius wants to explore everything the world has to offer and Pisces wants to feel everything the world has to offer, and those impulses actually move in the same direction more often than not. Say Sagittarius wants to book a last-minute trip somewhere neither of them has been. Pisces, instead of asking for a spreadsheet of logistics, just starts imagining what the light will look like there. That is the ease people miss.
Jupiter rules Sagittarius outright and co-rules Pisces alongside Neptune, so there is a shared appetite for meaning, for believing life is bigger than a to-do list. The friction shows up in how each of them processes an actual bad day. Sagittarius wants to talk it out loudly, maybe make a joke of it, and move on. Pisces needs to sit in the feeling first, and can read that Sagittarius humor as dismissive when it is really just how fire tries to lighten water. Sagittarius also says blunt things without softening them, and Pisces absorbs more than they let on.
What holds them together is a mutual refusal to be boxed in. Neither one wants a partner who keeps score. They just have to remember someone still needs to buy groceries.