what some call insanity

Yeah, I think Sarah's always had a habit of leaping before looking, from The Time Warrior on - obviously it pays off sometimes, but even in classic Who we see a lot of her going back and forth on her decisions (a la the way she kept threatening to leave the Doctor). Taking on Luke is an even bigger commitment, and after so long alone and what with her major abandonment issues, I think she'd have a really hard time once the initial euphoria wore off. I kind of wish it had been explored a little more on the show, but there's only so angsty you can get on CBBC, I suppose!

That said, she usually leaps into pretty awesome situations, and they certainly worked out in the long run :)