She bought it as a joke when they first moved in.
Soul was pushing her, insisting that she never did anything impulsive, thus somehow making her less interesting than watching paint dry. Maka's retaliation was that being impulsive didn't make anyone more or less fun, but her logic fell on deaf ears.
So when she saw the orange apron, she did what would embarrass any twelve year-old boy and bought the offending article for their kitchen.
His complaining and whining was music to her ears, and the way he cringed whenever he looked at it allowed her the satisfaction of pointing it out to him. Her victory, however, was short-lived. She hated it as much as he did; the frilly orange apron matched neither of their tastes.
They'd both go as far as to purposely stain the apron when wearing it just so that they would temporarily be rid of it. But it was a hassle; cleaning the unnecessarily large stains on the apron and the normal ones on their clothes on top of that was extra work. So they didn't bother anymore.
But for whatever reason, they never got rid of it. Maka had insisted that when either of them were cooking in the kitchen that they had to wear it. The garment was hideous, though necessary to protect the clothes underneath.
They never replaced it either, and as they got older they found themselves more comfortable with it. Seeing each other wear it made their situation seem more homey. Occasionally, they'd even make quips like "Oh, what's the wife cooking tonight" or "You made tea? Such a nice house husband." Neither told the other to stop.
Then the second came along, one that was baby blue with dark blue straps and trim. Maka had gotten it for Soul for his 15th birthday, which he deemed to be uncool (they already had one that they shared, why would they need a second?") Maka wasn't fooled so easily, he would always wear it when it was his turn to cook.
Some point during their partnership, they started crossing the line that separated friendship and romance. Light caresses when they were alone on the couch, a kiss here and there. It was when they finally had the talk that they stopped toeing that invisible line and dived all the way over to being lovers. When Maka became his girlfriend, she began "borrowing" his clothes, which included his baby blue garment. She wouldn't dare lose it; it was a gift from her that he became fond of. He wasn't upset (he rather liked seeing her wear his stuff.) Soul's plan at getting back at her was giving her a taste of her own medicine.
He was making dinner, wearing her frilly orange apron when he was caught red handed. But instead of demanding that he give it back, she just stared, her eyes becoming watery much to his alarm.
It brought a sense of nostalgia seeing him wear it. She got that for them to use not long after they had moved in together- over six years ago. And ever since Maka had gotten him the blue one, she hadn't seen him wear that frilly orange one since. So much had happened since they first became partners and the apron was a stark reminder of it.
She shouldn't be so emotional, it's just an apron. A piece of clothing that had stuck with them since they were preteens and used to wear. One that has loose threads, frayed with use, and stains they were never able to completely remove. But it was one of the things that brought them together.
Soul had set dinner aside momentarily to pull Maka into a hug, whispering sweet nothings in her ear while she babbled her recollections, unmistakable tear stains right on the seam.
After that event, they'd swap off aprons whenever they felt like it. Sometimes she wore her orange one and he wore his blue one. Other times she wore his and she wore hers. Both belonged to each other and they eventually stopped referring to them as "mine and yours." They loved seeing them wearing the aprons as they made food or watered the plants. But the occasions they enjoyed best were the times where one of them would wear either of "their" aprons and nothing else underneath.