Would a Matendou Sonata be complete without a page dedicated to the adorable relationship between Micael and Filla? Of course not! I love sweet mushy romance, and "Matendou Sonata" has one of the sweetest romances I've seen.
Filla loves Micael. That much is extremely obvious. He says so at least a dozen times per manga, and proves it by risking his life time and time again for Micael's sake. He even says that if he had to choose between Micael and the world and everybody in it, he would choose Micael. And, of course, he doesn't miss any opportunity to suggest that they kiss, jump into bed together, etc. In spite of that, he's not quite the pervert that Micael says he is. Even when he has the opportunity to take advantage of Micael, he refrains from doing so.
Micael is another story. The truth is that he cares about Filla just as much as Filla cares about him...but it's not always easy for him to show his feelings. He feels that he's doomed to hurt the people he loves, and he pushes Filla away because of that. It scares him when Filla risks his life to protect him. He has felt the pain that goes with losing a loved one, and he doesn't want to feel that pain again...even if the only way to achieve that is to never love anyone again. It takes him a long time, but in the end he does admit his true feelings to Filla.
Lucifer says that Filla only loves Micael because it was his destiny to do so. Filla disagrees, saying instead that he loved Micael even before he was born...and because of that, the gods were kind enough to give him this destiny. Is it love because of fate...or fate because of love? Either way, the bond between them is a strong one that is not easily broken.