Soul and Maka never said 'I love you' to each other. Not like their friends did.
Black*Star had first proclaimed to Tsubaki that he loved her in the heat of battle, she'd been in trouble and he'd screamed that the Kishin they were fighting could "never take away the love a man who was above God!" Tsubaki and Black*Star had begun to date after that. Black*Star would proclaim his love whenever and wherever he felt like, whereas Tsubaki would only whisper it to him; when he was feeling down, in the quiet of the night, in the throes of making love.
Ox always told Kim he loved her, and Kim always said the three words back. And she did love him. But she and Ox easily fell out of love once they began to date. It had started with small things, fights. Then it had progressed, they couldn't stand being near one another, they spent more time with their partners. Until they stopped staying 'I love you' all together. Then it was just a matter of time until they split up.
Kim made sure to tell Jacqueline she loved her at every chance she got. Little notes in her locker, decorating her notebooks and room with little hearts. Maybe she did that because she couldn't imagine losing Jacqueline like she lost Ox. Maybe she did that because she loved Jacqueline more than she loved Ox. Maybe she did that because she still couldn't believe that Jacqueline, her sweet lovely loyal Jaqueline, loved her back. Jaqueline only said 'I love you' when they were alone and when they first got together. Though as she grew older she told Kim she loved her more and more when she was drunk or when they were hanging up on the phone.
Kid and the Thompsons said 'I love you' plenty of times to each other, but it was never romantically. They loved each other like siblings. They loved each other like they were three parts of a single soul, a single essence. 'I love you' to them was like breathing, when it was said it was said offhandedly and frequently, but always platonically. Neither of the Thompsons could imagine being in love (or at least having sex) with Kid (or each other for that matter thank you very much), and (to tell the truth) it never even crossed Kid's mind, especially after he became Shinigami.
Stein and Marie first said 'I love you' when they were children. After all the time apart when they had that night together they said 'I love you' again. After Marie realized she was pregnant they said it more and more often. When they married they slipped it into sentences like it was asking to pass the salt, every chance they had they told each other.
Maka's Papa and Mama said 'I love you' all the time when Maka was younger. But after years of Maka closing her eyes and covering her ears as she lay on her bed to block out the yelling of her parents, the words 'I love you' seemed to lose meaning. Papa told Mama he loved her all the time, and yet it didn't seem to stop him from cheating. Papa told Maka he loved her too. She always wondered what he meant when he said 'I love you'.
But Soul and Maka never said 'I love you' at least not out loud. They're long romantic relationship started with a kiss, but no 'I love you'. When they were resonating it was simple, obvious, and just plain. They knew that they loved each other, so there was no reason to say it. But they knew. After every nightmare when one of them would crawl into the other's bed, saying sweet nothings into their partner's ear, they knew they were really saying, 'I love you'. After every battle, Soul would transform back and they would look at each other, and in that instance they knew that they were saying 'I love you'. It was in their eyes, in their body language, when they hugged, when they kissed, when they had sex. They were saying 'I love you'.