I think one of the things that attracts Sapphire to Ruby is that she surprises her. Think about it, Sapphire can literally see the future. Nothing could possibly surprise her. But Ruby is so unpredictable in both the good and the bad ways and Sapphire loves that. Like in this scene
Then Sapphire genuinely laughs and gets really happy. She really didn’t see that coming even though she can see the future. Even after thousands and thousands of years together, Ruby still makes Sapphire laugh and still surprises her.