Just about every romantic novel ever written and ALL Mills and Boon.
The alternative would be …
I really love you!
Good I really love you too!
Lets get married/live together?
Yes lets.
Novel finishes on page one.
I think I agree with you. I also remember learning that Dickens had to tack that sort of optimistic ending on because of his publishers but that he didn’t want them back together.
Mansfield Park
Anne of Green Gables.
Sam and Diane on “Cheers”.
Some Kind of Wonderful
The Fountainhead, kinda
Shrek
The Bride, kinda (yes, the Frankenstein movie with Sting as the evil Doctor and Clancy Brown as the Kind Monster- happiest Frankenstein movie EVAH!
Also has the best exchange:
She: You can’t tell me what to do! You didn’t make me!
Dr. F: As a matter of fact, I did. locks her in his library with his journals, waits for her to read & satisfactory listens to her SCREEEEAM!)
There’s an Italian film I just watched recently called (in English) “But Forever in my Mind” that sort of has this situation.
The main character is a high school student who has a crush on the girlfriend of one of his friends (I think the crush started before the two were together). They very nearly have a fling but things fall apart (primarily because he tells his best friend that they kissed and the friend lets the information slip and soon everyone knows, pissing her off considerably). In the aftermath, another girl, who is friends with both of them, tells him that she has had a crush on him for the last six months. He’s completely non-plussed and doesn’t at first return her feelings. He goes home and talks with his sister, who says he should figure out if he has feelings for her and tell her as soon as he knows. Then he talks with his brother who says that the best way to figure that out is to kiss her; if he kisses her and feels something it could work, if not then no. He runs over to her place and kisses her, discovering in the process that he has feelings for her. End film (except for a brief closing scene).
Surprisingly, I found this plot worked. I don’t know that it would have worked if they weren’t teenagers, but I found it believable and sweet for that time of life. Perhaps this wasn’t a perfect example of what the OP was looking for since once he learns of her feelings for him the unrequitedness lasts for maybe an afternoon (and a lot of the lag time could be ascribed to him thinking: “whoah, I didn’t see that one coming”).
Jules et Jim, for a different twist on it.
This is my college buddy Kevin. His wife liked him, he flirted, went away for the summer, didn’t do much with her when he got back, actually dated someone else for 10 months, started dating her, and now they have two kids.
That’s not true. There’s just as much stuff out there, if not more, where both of the leads fall in love at the same pace, or there’s an unrequited love involved, but the other lead has no idea it exists for most of the story.
Thanks for all the suggestions so far, guys! Keep 'em coming!
On TV there’s Moonlighting. Though it can be argued that the unrequited episodes and storylines are far superior to those once the love was returned.
Yeah, my first reaction to that objection was ‘what about all the stories where NEITHER party is in love with each other for the first half?’ 
Every single soap opera in the history of the world. Indeed, if I’ve learned anything from soaps, it’s that stalking someone who doesn’t want you will turn them around on the subject.
The Hotel New Hampshire if you don’t mind a little incest.
WALLE*. He won over EVE relatively early in the movie, but he was love stricken at first sight. She wanted nothing to do with him, even attempting to end his existence via laser pistol on numerous occasions.
Hannibal (the book version)
Lord of the Rings