[I normally don’t write "reviews", so, this is one of a few I’ve ever written. It contains a lot of spoilers and briefly analyzes the inner development of the two main characters, Keith and Natalie. I just want to share my thoughts for this movie is so beautiful, and my absolute favorite.]

The acting of both, Jesse and Elizabeth, is phenomenal. One can tell what the characters, Natalie and Keith, are thinking and feeling, and how those feelings develop as time passes. In less than 10 seconds it is possible to say which character feels what in any particular second.

Personally, I love mostly the development of both characters.

Natalie just wants to get her scholarship by getting good grates, doing extra work, winning tennis matches, and pleasing everyone around her. Then she meets Keith. She starts to question her goals for the first time in her life, and is slowly falling in love with Keith, although he is often trying to push her away (to protect himself or Natalie or both of them(?)), which is probably making her even more interested in Keith. Towards the end, Natalie clearly becomes as miserable and depressed as Keith, but she seems not to care too much about it. She chases after him, and despite her deep feelings for him, she lets him go. Both want a “chosen goodbye”.
At the beginning of the movie, Keith just wants to play with Natalie a little. He has nothing to lose anymore, and he hates her for her constant positivity and happiness. So, it seems that his final goal is to bring her down. He asks his chemistry teacher to be her lab partner, and then, he starts to manipulate her. He goes from ignoring her to stalk her, trying everything to get her to things she normally wouldn’t do. But slowly, Keith realizes that Natalie is actually an amazing person to hang with. Keith, too, is falling in love. At first, he is afraid of his feelings, because he before that he had accepted the fact that he was dying, but at some point, he wants to live just a little longer to be with her. He knows, he can't. He goes back and forth, pushing and pulling her, even when she finds out the truth.

However, LOVE IS A FORCE YOU CAN’T CONTROL.
Except, in the end, they both control it by saying goodbye earlier than they actually have to.
I, personally, think that the saddest part is right before the end, when Natalie tells Keith that they’re going to save bowling for the next time. There will (probably) be no next time, which makes her statement so heartbreaking.
All in all, I’m completely in love with the characters in that movie. The actors make it seem so believable and real, that I was really able to feel everything they felt. For me, this movie is a masterpiece and truly a hidden gem!