Here is the honest starting point: Sagittarius books a one way flight and asks questions later, while Capricorn wants the itinerary, the budget, and a return date before agreeing to leave the house. These two sit right next to each other on the zodiac wheel, which sounds cozy but often just means they are speaking different dialects of the same language. Saturn, Capricorn's ruler, is about consequences and long timelines. Jupiter, Sagittarius's ruler, is about expansion and possibility. When Capricorn is quietly building a five year plan, Sagittarius is already bored of year one and eyeing the horizon, and that gap can feel less like a difference of taste and more like a disagreement about whether the future is even worth planning for.
The friction shows up in small, recognizable moments. Capricorn will ask what time they are actually arriving somewhere, and Sagittarius will say "soonish" and mean it sincerely. Capricorn reads that as carelessness, Sagittarius reads Capricorn's need for structure as an attempt to cage something that was never meant to be caged.
But mixed modalities, cardinal against mutable, tend to balance rather than clash once trust is built. Capricorn gives Sagittarius a spine to build adventures on, a reason the wild idea might actually work if someone tracks the details. Sagittarius gives Capricorn permission to laugh at a plan gone sideways instead of gripping it tighter. Capricorn's earth grounds Jupiter's reach, Sagittarius's fire warms Saturn's discipline into something less severe. Neither will hand this to the other for free. Both have to choose it, repeatedly, on purpose.