It's only natural
There were no flowery declarations of love. No "confessing", no bashful glances or shoegazing and toe scuffing. There might have been some fierce blushing (which is really to be expected when you get your first kiss from your best friend), but there were absolutely no soft giggles or moony eyes.
Their first kiss was, in a word, disastrous. They had been shouting, and in each other's face, and she remembered being just incredibly angry. When their lips crashed together, he bruised his lip, and she cut hers slightly on his teeth. They pulled away, eyes wondering, hearts pounding, and when they eased back together, it was soft and hard and wet and good.
Maka didn't understand at first. She had read all about relationships in the books that she'd taken from her mother's "special" shelf. But nowhere did they mention that you could have a relationship with someone and have nothing really change between you. It bothered her at first. At least until she realized that she really didn't want anything to change between them.
They already had a relationship. They walked to school together, sat together, ate together. Hell, they lived together for that matter. Soul already knew when her birthday was, and if he got her something a little nicer now, well, that was hardly much of a change. They already fought, with each other and at each other, so that wasn't anything new, either.
The biggest difference, she thought, was the physical. They'd always been rather touchy with one another-Soul especially. Little touches on her shoulder, or on her lower back evolved into covert butt-squeezes and arms wrapped around her waist. Maka enjoyed holding his hand as they walked to and from school, or as they sat wrapped together on the couch. Soul would roll his eyes and say something about how completelyuncool it was, but he'd always squeeze her hand as he grabbed it.
When she realized that she loved him, it wasn't dramatic. It wasn't life-altering or soul-shattering. They were sprawled together on the couch, her with a book, he with a game controller. She'd looked up from her book to find that he'd paused the game to wrap his arms around her in a display of unexpected affection. She leaned back into him, surrounded by his smell and warmth, and knew that this was love.
She asked him once when he'd known that he loved her. Soul blinked at her lazily and shrugged as he shoveled more pie into his mouth.
"Since always," he stated matter-of-factly.
There were times, especially after they began to share a bed, that Maka would begin to worry that she was doomed to repeat the mistakes of her mother and father. When she said as much, Soul would raise an eyebrow and ask her if she was on the rag.
When he had recovered from the spinal end of a book to the head, he'd wrap an arm around her, even as she tried to shrug him off.
"We're not your parents," he'd whisper in her ear, and slowly she'd relax into his arms, feeling the truth of his words.
It was natural. Like any relationship, it wasn't always easy. But Soul'd been dealing with her neuroses for years already, and though sleeping together brought new neuroses to the forefront, it wasn't much of a struggle to handle those either. He might have even caught himself thinking that the way she freaked out over things was a little cute. He also thought that he might have brain damage, but that was neither here nor there.
There were times when she was certain that his devil-may-care attitude was going to drive her completely insane. Or the few times his family had come up in conversation, and she'd had to simultaneously calm him and get the subject changed. His pride was easily wounded when she'd laugh at him, which was frequently, but she was okay with making his favorite meal to make up for it.
They were partners first, then friends, and then lovers. For Soul and Maka, it was an almost seamless progression. Nothing about them had been easy, or smooth. They fought just as much after they'd been together for five years as they had when they'd been together for six months (though Soul really enjoyed being able to engage in some phenomenal make up sex). She still did stupid, rash things, and he still berated her endlessly for being such a nerd, but at the end of it all, they knew they'd still go home together, wake up together, live and breathe together.
It was rarely easy. But it was natural, and they wouldn't have it any other way.