People assume this pairing is all fireworks and no follow through, two fire signs burning bright for a season and then burning out once the novelty wears off. What actually happens looks different from the inside. Leo and Sagittarius do share that fire, but Sagittarius is mutable and restless while Leo is fixed and rooted, and that difference turns out to be the thing that keeps them together rather than the thing that ends it.
Think of them as a stage and an open road. Leo wants a place to shine, an audience, a sense of occasion. Sagittarius wants to keep moving, to see what is over the next hill and report back with an honest, sometimes blunt opinion about it. Put them together and you get a relationship that rarely sits still: last minute trips, big dinners with too many people invited, Leo dragging Sagittarius to something dressed up and formal, Sagittarius dragging Leo to something with no itinerary at all. Both sides tend to enjoy it more than they expected to.
The friction shows up around ego and honesty. Leo wants to be adored specifically and often. Sagittarius means well but will say "you looked better in the other outfit" without softening it, and Leo can hear that as carelessness rather than candor. Leo, in turn, can come across as needing constant reassurance, which grates against Sagittarius's need for open space and no obligations.
What holds it together is warmth and generosity on both sides, and a shared refusal to be small or petty about life. If Leo learns to laugh off blunt comments and Sagittarius remembers to actually say the nice things out loud, this pairing has real staying power, loud, cheerful, and rarely boring.