It's hard to know what Maddie really wanted from David because, you're right, she never tells him what she wants. He asks her what she wants, and tells her he will do what she wants, but she never comes through with anything. I don't think she knows what she wants from David.
I do think she wants to be in control. She's been alone her whole life... an only child, no close friends, no long-lasting relationships. Being with David, even though she may be madly in love with him, threatens her ability to have total control over her life.
I think that's why she says, "If this is love, it's too hard. If this is love, I'll live without it." Ultimately, she always chooses to have "control" over "love".
I think if she had given him more of a chance, Maddie would have seen that, with David, she could've had both. As detectives, he worked with her and her controlling ways for three years, all the while falling more and more in love with her.
David just needed to know she loved and wanted him in return. I think, within reason, he was okay with her being controlling.
Spoken by a self-proclaimed control freak over here.