Why assume that she'll get Saito?
I mean, theoretically she's supposed to get the familiar that's most suited to her, or something like that.
So what happens when someone changes the character of the summoner by, say, greatly reducing her inferiority complex, giving her a better sense of self-worth and teaching her to treat everyone - including those far below her own station - with respect? That's right, there's a very high chance that another familiar will be better suited to them.
In this case, a Louise that actually likes her explosions would probably end up summoning someone who knows explosions. There's many ways to interpret that, but Saito? He knows nothing about explosions (that an average teenager doesn't know).