What each one finds fascinating
Sagittarius is everything that Scorpio doesn’t let themselves be: open, unguarded, sure the world will catch them. Scorpio is everything Sagittarius can’t work out, and that’s catnip for a sign who loves a big question. Both share an interest in what’s true, and neither is much good at small talk. Scorpio wants the truth about people, and Sagittarius wants it about the universe. Late nights tend to be very good.
Privacy and the open door
The recurring trouble is about what stays between you. Sagittarius shares things casually, because to them secrets are mostly a burden. Scorpio takes that as a betrayal, and may not say so. Instead they go quiet and start to withhold, which is the one thing that makes Sagittarius feel trapped, so Sagittarius books a trip. Scorpio watches them pack and draws conclusions.
Their tolerance for intensity is different too. Scorpio wants to go to the bottom of the fight, and Sagittarius wants to shake it off and get a burger. Each thinks the other isn’t taking it seriously, or is taking it far too seriously.
Agreements that actually work
Be explicit: this story is ours, that one you can tell. Sagittarius can come to see that being trusted with a secret is a form of being loved. It’s also better to announce your need for space than to vanish: “I’m going for a long run, I’ll be back at six, and we’re fine.” Scorpio can try to enjoy the good news when it’s happening, and let a free afternoon be just free.
Neighboring signs like these usually need help from elsewhere in the chart. Mars in Sagittarius or Scorpio would tell you who starts what. And some Scorpio Suns near the very end of the sign have planets in Sagittarius anyway, so they might not feel so foreign.