Water beside fire
Scorpio conceals as a matter of policy, on the reasoning that information given away cannot be recovered. Sagittarius says the thing, immediately, to whoever is present.
Each finds the other's habit close to a moral failing. Scorpio regards Sagittarius as indiscreet, and it is right. Sagittarius regards Scorpio as unnecessarily guarded, and it is also right. Where the pairing does well, Sagittarius drags Scorpio into daylight and Scorpio teaches Sagittarius that some things are worth keeping to oneself.
Fixed and mutable
Scorpio is fixed and Sagittarius mutable, which produces a mismatch in duration. Scorpio holds a feeling at full strength indefinitely. Sagittarius has usually finished with the matter by the following morning.
The practical effect is that Sagittarius does not know an argument is still live. It behaves normally, Scorpio reads that as indifference to something serious, and the original disagreement is compounded by a second one about how seriously it was taken. This recurs, because it is a difference in modality rather than in attitude.
Mars, Pluto and Jupiter
Scorpio is ruled by Pluto in modern practice and Mars in the traditional scheme. Sagittarius is ruled by Jupiter. Mars and Jupiter are both active bodies, which is why this pairing is rarely dull, and they act on completely different timescales.
Jupiter enlarges and forgives easily. Pluto examines and does not let go. Set against each other they produce a pairing in which one partner has moved on from an episode the other is still, quietly, working through.
Where it grinds
Trust and its verification. Scorpio wants to know where the partner has been and Sagittarius resents being asked, since being asked is the thing it least tolerates. Neither concedes, because for Scorpio the asking is due diligence and for Sagittarius it is confinement.
Money is a related problem, since Jupiter spends and Pluto wants an accounting of where it went.
The second friction is tact. Sagittarius makes a careless remark about something Scorpio has never discussed with anyone. It is forgotten within the hour by one party and not at all by the other.
Why the suns do not settle it
Neighbouring signs are conventionally described as incompatible, which is a statement about averages across a very large population and not about any two people. Plenty of these pairings last, and they do so on the strength of placements the sun sign does not show.
The Moon determines how each behaves when hurt, which is the operative question here. Venus determines how affection is given, Mars how conflict is run. Those three, compared across two charts, are where a real answer sits.
Questions people ask about this
Are Scorpio and Sagittarius incompatible?
They share no element and no modality, so nothing comes easily. That describes a tendency across a large group, not a verdict on two people.
What is the central problem?
Disclosure. Scorpio withholds by default and Sagittarius says everything, and each regards the other's habit as a failing.
Why does Sagittarius seem to dismiss Scorpio's feelings?
Mutable fire finishes with a disagreement quickly. It is not dismissal, though it reads as dismissal to fixed water.
What matters more than the sun signs?
The Moons above all, since the pairing turns on what each does when hurt, then Venus and Mars.