Love is the most often recurring theme in anime. Actually, it occurs so often its not funny. Every friggin show out there that you see is chock full of the words "suki", "aishiteru", "dakishimete" etc etc etc words of romance. Take -ef- for example. Their script was probably written by a little girl smitten with fantasy stories that came out of Enid Blyton... hold it, before you open your mouth any larger, I'm just ranting here. I know ef came from a kickass hentai game... 's ass.
Romance is often used to back a story up much like any other plot device. Making characters unsure and unaware of their relationship makes it all the more true to life. This is one of my favorite scenes from Myself;yourself that really put me on the edge of my chair. Well, not quite, but it did make the anime all the more exciting. I mean every show has got to have some of this stuff right?
It seems that whenever a character in an anime gets featured in a romantic relationship, no matter how unpopular they would be, their ratings naturally shoot up. A prime example of this is Nagisa from CLANNAD who simply isn't the most popular girl in the anime and might as well become another girl-who-sits-in-the-resource-room if not for her redeeming relationship with Okazaki. This goes to show that love is indeed a strong basis for a plot, and the strange mechanics of the human mind simply makes those who are in love popular.
Of course, sometimes love is also used as a device to smash the story into the face of the audience. Shock therapy is conveyed via love in Suzumiya Haruhi no Yuuutsu's last few scenes where Kyon kisses Haruhi. Despite not being quite the classic example of romantic indifference in anime, it certainly brings to life the characters by reminding people of how love in real life constantly exists in this form. Of course, the climax, long awaited for by all those who anticipate it, make the story all the more rewarding if it happens just at the end.
Love Hina used the same technique by constantly delaying the final makeout scene to the very last episode of the Love Hina Again OVA. The moment it happened, I could've sworn I went "AWWWWWWW" in my head.