Rarity: The unicorn mare walked across the street of the slow moving Ponyville, her mane a bit of a curly mess, makeup barely done, and eyes tired from crying. The only reason she had gone outside at all was because she had to take care of her daughter by herself, and had no-pony else to go shopping. As she approached the market she heard her name being called, and she turned her head slowly. “Hmm?”
???: “Rarity!”
Rarity: The mare turned and saw the stallion and blinked. “D-Double Diamond?”
Double Diamond: “How are you, Miss Rarity?” He slowly approached her, causing her face to heat up slightly. “How have you been?”
Rarity: She paused and felt like it was a bit of an odd question, seeing as she looked like a mess. “I-I…Uhm…Ehehe…” She tried to smile weakly, not sure what to say. “I suppose I’ve been doing alright.”
Double Diamond: He paused and tilted his head. “That didn’t sound very confident…Is something wrong?”
Rarity: “Oh, darling I won’t worry you in my own family problems.” She smiled weakly and sat down on the ground avoiding a bit of eye contact.
Double Diamond: The stallion smiled softly and gently placed his hoof close to hers, just to reassure her. “I’m not in any hurry. If you’re having a rough day I’d love to sit and listen.”
The mare sat down and couldn’t help but to explain him about her recent divorce with her husband - Fancy Pants - and her struggle with being a single mom now. The stallion didn’t speak most of the time while she did, and just listened. By the end of their conversation, Rarity had talked for about an hour with the stallion and he smiled, wrapping his hooves around her. The two hugged it out and she finally smiled a genuine smile.
Double Diamond asked if she had been living in Ponyville and she nodded, replying that she and her daughter Opal had been staying at her old Boutique, since it was still in business. The stallion asked to see her again, and she agreed. The two left and returned to their own homes, before their friendship started and soared. Double Diamond became one of her closest friends, finally getting her out of her slump, and even Opal seemed to love the stallion. He was more of a dad than her own father was.
About a year after they met, Rarity and Double Diamond actually began to engage romantically, and later on married. Rarity loved having him around. He really was her best friend.