Water and fire, unevenly matched
Fire warms and water contains, and in this combination each tends to spoil what the other is doing. Sagittarius states things at full volume without weighing them, and Cancer, which weighs everything, takes the damage.
Meanwhile Cancer's caution reads to Sagittarius as pessimism. The pairing does have a real benefit: Sagittarius pulls Cancer out of a shell it would otherwise stay in, and Cancer gives Sagittarius somewhere to return to that is not just another destination.
Cardinal and mutable
Cancer is cardinal and starts things with a view to keeping them. Sagittarius is mutable and treats arrangements as provisional. That asymmetry is more disruptive here than the elements are.
Cancer wants an answer about the future because the answer is what allows it to relax. Sagittarius genuinely does not know and is untroubled by not knowing. Asked directly, it will give a warm and non-committal reply, which Cancer stores and later finds was not a commitment at all.
The Moon and Jupiter
Cancer answers to the Moon, which governs habit, memory and the need for familiar surroundings. Sagittarius answers to Jupiter, which expands and dislikes a boundary of any kind.
These two are not naturally hostile. Jupiter is a benefic in traditional terms and the Moon is receptive, so the pairing is often generous and easy in temperament. The problem is scale rather than tone. Jupiter enlarges the territory and the Moon wants the territory small enough to feel safe in.
Where it grinds
Bluntness against sensitivity. Sagittarius says the true and unkind thing without registering it as unkind, and Cancer does not raise the injury, it withdraws. Sagittarius then notices the withdrawal and not the cause.
Sagittarius will also promise more than it delivers, not from dishonesty but because Jupiter agrees in the moment and revises later, and Cancer keeps the original version.
The second friction is roots. Cancer counts the number of nights spent at home; Sagittarius counts the number of opportunities declined. Both are keeping score of the same year and arriving at opposite conclusions about who is conceding.
The suns are the least of it
This is one of the pairings that reads badly on paper and turns up constantly in real life, which is a reasonable indication of how much a sun-sign comparison actually settles. A sun sign describes a tendency running through a very large population.
The Moon is where an emotional match is decided, and one partner here is Moon-ruled, which raises the stakes on that placement. Venus shows what each offers, Mars shows how they fight, and those are the placements to compare.
Questions people ask about this
Can Cancer and Sagittarius live together comfortably?
It depends on how much travel and unpredictability the Cancer partner tolerates, and whether Sagittarius accepts a fixed base.
Why does Sagittarius keep hurting Cancer without meaning to?
Sagittarius says true things without weighing the delivery. Cancer absorbs delivery as much as content, so the same sentence lands harder.
Do the Moon and Jupiter conflict?
Not in tone. Both are warm. They disagree on scale, since Jupiter expands the field and the Moon wants it contained.
How much does the sun sign decide?
Very little on its own. Compare the Moons first, then Venus and Mars, which is where the real differences show.