Sagittarius and Capricorn may understand rules differently. Sagittarius wants to define them and put them into words; Capricorn may accept them with apparent ease and later adapt them in their own way, sometimes without Sagittarius noticing. What one understands as a fixed rule may be, for the other, a framework that allows adjustments.
Social life is another delicate point. Sagittarius needs friendships and frequent get-togethers. Capricorn may take part when those activities relate to their goals, but also wants to protect their privacy and withdraw when they need to.
It may be hard for Sagittarius to understand that withdrawal and respect it without feeling left out. Talking about when they need company and when they need privacy helps keep autonomy from being read as a rejection of the relationship.