Ok, first of all: thank you all very much for your incredibly quick and helpful answers!
I've just tried it (as most people here, I just happen to have a savegame before every conversation with him

) and although I don't know why, I'm still not getting this line. I used the right conversation (as siriusrambles said, the one after the last mission before Ilos, it starts with "We're so close now"), used the upper right Paragon options and I did get Kaidan's
"No bull, Shepard. I wanna follow through with this. It's tough
keeping it separated from duty. But when the mission's complete, it'll
be different. I hope you feel the same way. Ma'am."
But then, picking the middle option (Absolutely) my Shepard says
"I hear you soldier. I'm in agreement." (instead of " I’m waiting with bated breath.")
to which Kaidan answers
"Glad to hear it."
Why can't my Shepard sy the line she's supposed to say, so Kaidan can say the line he is supposed to say?! *sobs*
Seriously though, does anyone know what's causing this?