Because... [ Grrr.. she doesn't know how to answer this without lying or yelling or resorting to her normal argumentative ways. ]

Because obviously you needed it. And we're partners, so long as no one else is here. No Satsuki-sama, no underachiever, none of the other student council members.

[ She pushes her shoulder into Ryuko. ] So when you're moping around because you can't see your other friends, I was just getting tired of it, that's it!