At a young age, Rarity always thought that Applejack was pretty. Sure, Rarity didn't like it when Applejack gets all dirty from work, but that doesn't completely deter Rarity from still thinking Applejack was quite pretty.
Sometimes, Rarity would attempt to make Applejack still look pretty even through all that dirt.
At age 13, Rarity started the project by giving Applejack a necklace. She was slightly disappointed when Applejack told her that she lost it somewhere in the field, but Rarity just told herself that the necklace wasn't that glamorous anyway. Besides, the necklace would had gotten dirty too, so back to the drawing board.
At age 14, Rarity decided to make this simple by making Applejack wear a bandanna around her neck. Rarity thought it looked cute on Applejack, but the idea was thrown away when Applejack attempted to save Opal from a tree. Applejack claims that Opal might be evil because Opal swiped at her neck, but her bandanna saved her. Rarity claims that's Opal's way of saying that Applejack didn't look good with the bandanna on.
At age 15, Rarity thought a pair of apple-shaped bobbles would look great in Applejack's hair, and they did. Sadly, the pigs thought that they look so good that they thought they were edible. The Apples had to take that pig to a vet right away. Luckily, the pig survived, but for Rarity, it was back to the drawing board once more.
At age 16, Rarity had attempted to continue on this project by placing a bright red feather in Applejack's hat. It's spiffy-looking until Applejack had to clean the pig pen. Instantly got dirty once the earth pony step a hoof in there. Rarity had to make a note to herself to make something pig-proof for the next accessory.
When Rarity became 17 years old, she decided to give Applejack some hairpins. They were simple, cute, and they help push back Applejack's mane when she works. It has to work, and they did for a while, which Rarity felt like it was an accomplishment for the project. One day, Applejack had told Rarity that the hairpins were working just fine until she found out that Rainbow Dash was attempting to break into the cider cellar by using Applejack's hairpins on the lock, so she have to give back the hairpins to avoid this even ever again (yeah, sure, Rainbow can buy her own hairpins, but still...). As Applejack took Rainbow to Cider Anonymous, Rarity felt like she should give up.
Rarity was 18 when she decided to continue on the project. She decided to go back to her old roots of the project, and decided to give Applejack a pair of earrings. Rarity was certain that nothing could go wrong with a pair of earrings, but when she approach Applejack, she noticed how flustered the earth pony looked when she saw the velvet box. It clicked in Rarity's mind that Applejack might like her more than a friend. The way she reacted to the velvet box, the blush staining her cheeks, her disappointment when Rarity told her they were just earrings. Applejack likes her, and Rarity might like her back. She decided to not give Applejack the earrings, and instead decided to give herself some time to think about this.
Did Rarity like her more than a friend? The whole reason for the project was because she wanted Applejack look beautiful even when she was working at Sweet Apple Acres. Did she just like Applejack for her looks? No, Applejack is a lot more than just looks. She's hardworking, dedicated, determined, honest, and caring. What's not to like about her even with some of her flaws? Does she like Applejack more than a friend? Rarity thinks so, and she can't believe she haven't realized this sooner. She likes Applejack, and Applejack likes her, so she was certain it might be good.
When Rarity was 19 years old, she finally has an idea on how to ask Applejack out.
"Huh, almost thought she've gave up on that," Applejack said to herslef. "But wouldn't she think this one more... throughly?"
While she's working, Applejack notice something on one of the barrels, which were a gold crown decorated with green gemstones, and a note with three, blue diamonds in the corner of it. In the note, it simply said to bring the crown back to the boutique when she was done with work.
Applejack knew there must be something more than bringing her crown tot he boutique, but what? It couldn't be that bad though.
When she arrives at the boutique, Sweetie Belle stood next to the front door, dressed in a pink dress and a little, silver tiara.
"Sweetie Belle?" Applejack can't help to raise an eyebrow at the little unicorn.
"Hi, Applejack." Sweetie Belle said, then let out a giggle. "I mean, hello, Miss Applejack."
"...What's Rarity's plan here?"
"I'm not suppose to tell." The little unicorn opened the door. "Come in, come in."
"Alright, but if she thinks she's going to make me wear a dress while I work..." Applejack started out at first, but upon entering the shop, Applejack noticed it was dimly lit with candles lighting up the area, guiding her way to the kitchen where Rarity sat at a table.
In her simple, white dress, there was a hint of nervousness on her face until she saw Applejack come in. A smile blooming across her face.
"Applejack, you must be wondering why I want you here." Rarity said.
The earth pony place the crown on the table, between the two. "Yeah, I do."
Seeing the blush appearing on Rarity's face made Applejack's heart race. "Well, do you remember how I try to give you a little... extra something? Right?"
"Yeah, I've always reasoned that you just want to make me pretty."
"It's true, but I've realized a year ago that you like me," Now it was Applejack's turn to have her cheeks turn pink. "And I realized I like you too." Now they've turned pink.
"W-What?"
"You see, I've always thought you look pretty when we were young, but I was trying to make you look pretty while you work. I've never actually understood why I was determined to continue that project until I just realize you like me, I realized I like you more than a friend," The crown glowed light blue, floating a little. "And I hope you don't mind me asking, but would you go out with me?"
Applejck paused for a moment. "You truly like me?"
"Yes, yes, I do. I-I want to give us a chance. After years and years of not realizing it, I want to have a chance to love you."
The crown rested on Applejack's head.
"...I really want to give us a try too," Applejack said, reaching over to grab Rarity's hoof. "And I promise that I'll make you have the happiest mare ever."
Just like she promised. She did everything she could to make Rarity happy while Rarity did the same for her.
When Rarity became 20 years old, she decided to finally end her little project with one last accessory: A wedding ring.