Why I think the “I love you” in response to “what do you need” is so powerful:

In a way, Elliot is thanking her and recognizing all that she is sacrificing to be there for whatever he and the kids need. Olivia, despite initially telling him that she “can’t” go back to whatever relationship they shared before he left,

is putting her own feelings of hurt and vulnerability aside to be there for her partner, telling him she will do whatever he needs. She is CHOOSING not to focus on how he left for 10 years without telling her, but is instead invested in his well-being.

And I think he is realizing that she is sacrificing her own emotional & physical health for him and is truly telling him she loves him through her own self-less actions.

So when she asks him “what do you need”, he realizes the full weight of her love for him and his love for her in return & responds accordingly.