"I can't. The door doesn't exist for me. It does for Tali, but she doesn't want to leave me on my own here." Shepard can't understand why, although if their positions were reversed she'd be doing the same thing.

Shepard tells him the date and time - Citadel-side - of her first entrance to Milliways and that of her second entrance, the one she got Bound on. "If you're from after that point, don't tell me or Tali about anything that's happened since. I'd rather not know."