Makoto Shinkai

If you don't recognize that name, then you need to become immediately acquainted with him for you own good. Here is a simple Wiki for quick orientation. It is one of my missions in life to introduce him to as many people as possible. I've even shown Voices of a Distant Star to my mother.

Makoto Shinkai has 3 major creations and one small short. His first cartoon, titled She and Her Cat, is a very short piece that follows a cat's daily life, all from the cat's perspective. It's insanely adorable, but also rather melancholy. The cat's master seems to be depressed very often.

His second work is a roughly 20 minute movie called Voices of a Distant Star. (She and Her Cat can be viewed on the Voices DVD as an extra feature, just FYI). Voices was created entirely by Shinkai (as was She and Her Cat) on his Macintosh computer. You can also enjoy a wonderful vocal dub on the DVD by Shinkai and his wife (there are only two characters in the film), though a professional Japanese dub was also done, as well as (of course) an excellent English dub.
Voices follows the story of Mikako and Noboru, two young students (I think they're about 15 at the start of the film). Mikako gets drafted into their army to become a mecha for the space ship Lysithea, but Noboru gets left behind (Mikako's grades are better, etc). The only way they can keep in touch is via text messages with their cell phones. Mikako sends these almost daily, but as the Lysithea travels further out into space, the messages start taking longer and longer to reach Noboru, with one taking over 8 years to reach him.
The story is beautiful, honest, and emotional. And the animation is truly breathtaking. The stark contrast of the hand drawn looking characters in their CG environment is a major part of what makes Shinkai stand out as a master in animation.

His second film, Beyond the Clouds/The Place Promised in Our Early Days, is something I can, unfortunately, tell you less about. I only saw it once or twice, and I wasn't immediately taken with it. Shinkai's stories are served better in an episodic format rather than a feature film, as Beyond the Clouds is. There are several jumps in time within the story that probably required more attention on my part that I gave the film at the time. But I felt that Voices was far superior, though this film was by no means a bad film. There are also two boys in this film, Hiroki and Takuya, and I recall having some trouble telling them apart as they grew older. And then there is the girl, Sayuri.
The three teens come across a crashed plane and decide to work on it together, so that one day they can fly across the water and visit Hokkaido Tower (a large, mysterious tower being built by the northern military faction, the Union). The strange tower has some sort of ability to change the matter around it and form alternate realities. Somehow Sayuri becomes involved with the Union and falls into a coma where her mind is trapped in a parallel universe. Her coma is directly related to the effect of the matter changing tower. Takuya joins the National Security Agency, and the Uilta Liberation Front (which is working to destroy the tower). Hiroki, who was in love with Sayuri, lives in constant depression without her. The two boys must eventually unite to save Sayuri or save the world.

5 Centimeters per Second, Makoto's most recent film, hits his usual themes of time and distance hard core style. The film's title references the speed at which cherry blossom petals fall. This film follows Takaki and Akari. Close friends in elementary school, Takaki and Akari are forced apart when Akari's parents move to Tochigi Prefecture for their work, while Takaki attends school in Tokyo. They keep in contact by writing letters, and although they clearly have feelings for each other, the time and distance between them works against them. When Takaki's family decides to move to Kagoshima, Takaki realizes that he will live too far away to ever visit Akari, so he decides to take a trip to see her. Unfortunately the night he leaves is wrought with heavy snow and his trains are consistently delayed. His heartbreak is, well, heartbreaking, as he begins to realize that he may not get to see her one last time. She managed to wait for him at the station, and the two talk and finally share a kiss. But there is a sad, heavy feeling, as they know they will only grow farther apart.
The film is divided into 3 sections, each running about 20 minutes. The first, Cherry Blossoms, was just described. The second, Cosmonaut, again follows Takaki, in high school now. A classmate named Kanae has feelings for him, and the episode centers mostly on her and her thoughts than on Takaki himself. But all the same, we see Kanae trying in vain to gain his affections; Takaki always seems to be lost somewhere else.
The final segment, 5 cm per Second, wraps the story up as the characters go their seperate ways. Takaki now works as a computer programmer in Tokyo, but he seems perpetually hung up on the past. Akari on the other hand has moved on and is getting married. It leaves one to wonder what would have happened had Akari and Takaki not been forced apart.
Again, another beautiful, honest, and absolutely heartbreaking piece from Shinkai.

You've probably noticed some obvious themes there. Shinkai seems relatively happy in his interviews. He's happily married, he enjoys working on his animations, etc. So I'm not sure how he's able to write such sad stories about love, distance and time. But it's best not to wonder about the creator and just enjoy the films, and let the emotion present in them wash over you. He gets hailed as "the next Miyazaki" a lot, but I think the honesty and emotion that Shinkai puts up on screen is a bit different than what Miyazaki creates. Not any better or any worse, just a different sort, presented in different ways. I strongly encourage checking out, at the very least, Voices of a Distant Star. If you're prone to "becoming emotional" at all, make sure you have a box of tissues nearby, because you will probably cry. I bawled my eyes out the first time I saw it, and I still shed some tears every time I watch it.
It's not that anything horribly tragic happens in Shinkai's films, but (and I know I keep using this word) the honesty of the characters' emotions is really moving.