Disclaimer: The fact that I do not own Harry Potter has not changed.
For her it was realizing she was in love with him. He was her idiot now and she felt she would be unable to live if anything happened to him.
The words had come out without her meaning them too after he had returned, injured but alive, after a particularly dangerous Order mission. He looked dead on his feet, blood was soaking through his shirt. He apparated back into Sirius' flat in the middle of the night and his eyes had darted around desperately, looking for someone. His eyes landed on her and he seemed to sag with relief.
She had been sitting up and Sirius had quietly offered her a drink and kept vigil beside her, anchoring her to sanity.
"James." She breathed in relief, throwing her arms around him. He grunted in pain and half collapsed on her and she was whispering "I love you, it's alright, I love you" without meaning to.
He pulled back from her and smiled so wide it was a wonder he didn't split any cuts. "I love you too."
They fought beside each other many times, facing Voldemort himself together three times. They kissed desperately before every mission for luck and found solace in each other whenever they lost someone important.
Everything was terrifying but he was her rock, her light in the darkness. She felt it only right to tell him. Soon after that they had moved out of Sirius' flat, who was happy to be able to entertain guests without them present but missed them dearly, and found a small place for just the two of them. They had a party with the remaining of the ten Gryffindors from their year plus some friends they had made in and through the order.
For him it was finally telling her he was in love with her. It was a relief to finally be able to say it whenever he thought it and not supress how he was feeling. He said it to her so many times he lost count but the words never lost meaning.
Of course, they still argued but it ended much better than it used to. The marauders frequented their small flat, Sirius was there nearly every day, and they fought evil and made a life together.
Then it was Frank and Alice's wedding and she was the maid of honour and she had never looked more beautiful. He danced the night away with her and their friends and truly relaxed and had fun for the first time since graduating Hogwarts.
They drank and murmured sweet nothings and engaged in general merriment, finally disappearing shortly after the bride and groom went on their honeymoon.
A couple of days later he found himself with Remus and Sirius in a ring shop (Peter was at home with his extremely ill mother). He carried it around with him everywhere but didn't know when or how he was going to propose to her.
He told his parents' gravestones about her and missed them terribly. They should've have been around for this. Should've made it to his wedding. His Dad should have given him advice and his Mum should have given him her engagement ring. It had followed her to the grave and he wouldn't dig them up, couldn't dig them up, for just a family heirloom. So Lily got her own special ring.
Sirius needled him about being best man and Remus assured him he had to pick Sirius. Peter was increasingly absent which worried him because he felt bad for him. He hoped his Mum got better, it hurt like hell to lose your parents.
He found himself trying to find an opening to ask her, constantly irritating her by not listening to a word she was saying. She always looked oddly at him when he did this because he usually paid attention to everything she said.
He loved her and wanted to be with her for the rest of their lives, but this had to be special and he found himself, for the first time in his life he was sure, lost for words. He knew it would be alright because he loved her more than words anyways.