Remus Lupin always knew that Alice Fawley was not for him, always knew.
And as much as it hurt him, he was there for her.
He was there at her wedding to Frank Longbottom, when her son Neville was born, and more importantly...
He went to see her when he found out the Lestranges attacked her.
As much as it hurt him to see her in that state.
With all of that, he was never able to let her go, but now that Tonks had come into her life, filling of color everything he thought was gray, he realized that he could finally let Alice go.
It is not my creation, it is JK Rowling's, and the whole universe is created by her, I only change small facts for my story.
Also, this is a translation of the original version (I wrote it too), but as English is not my first language please correct me if I got something wrong or if I can improve.