If Hori is from a particular anime or book, we would need more details about that source. Maybe it's about a high school student Hori, but still, we need to know more about the backstory of her or his parents to tell their love story.
Another key event could be when they faced some sort of obstacle in their relationship. It could be family opposition, or maybe a long - distance situation if they had to be apart for some reason. Overcoming these difficulties would be a significant part of their love story. If Hori's parents' story is a common one, they might also have a romantic proposal and a beautiful wedding as important events.