In the ending of the 'My Happy Marriage' novel, it could be that the main characters have a deep realization about their relationship. They understand each other's flaws and strengths better. For example, they may have faced some misunderstandings throughout the story, but in the end, they communicate openly and resolve all issues. This leads to a more profound and lasting love, which is a great way to end a story about a happy marriage.
Typically, there's the element of love at first sight or the gradual development of deep affection. For example, the characters might meet in an unexpected way and feel an instant connection. Another common element is the overcoming of obstacles. It could be family disapproval, like in many classic stories, or some personal issues the characters have to work through. Also, there's often a moment of realization where the characters understand their true feelings for each other.
It's a novel. The structure and style of My Happy Marriage indicate that it originated as a novel, not a manga. The depth of character development and the complexity of the plot are typical of a novel format.
The ending might conclude the themes of love and sacrifice. If one character has been sacrificing a lot for the marriage, in the end, they may see that their sacrifice was worth it as their relationship has become even stronger.