"The One Thing"
The title of this fanlisting comes from that quotable exchange between Tony and Pepper in Iron Man 3 (2013). Thanks to IMDB for the quotes below:
PEPPER: Oh, really? I didn't notice that at all.
TONY: You experience things and then they're over, and you still can't explain them. Gods, aliens, other dimensions. I'm just a man in a can. The only reason I haven't cracked up is probably because you moved in. Which is great. I love you. I'm lucky. But honey, I can't sleep. You go to bed, I come down here. I do what I know. I tinker. I... Threat is imminent. And I have to protect the one thing that I can't live without. That's you.
So there's actually a parenthetical "I can't live without" after the titular phrase I adopted. But choosing to make the main title elliptical provides the ambiguity that I rather like; you can continue the quote with your own phrase, depending on how you view Tony and Pepper's relationship, and
hopefully it will still ring true. ;)