One important aspect could be love. Without love, it's hard to have a truly happy marriage. Another might be trust. If you trust your partner completely, it can make the relationship very strong.
I don't know your happy marriage story. You can start by telling me how you two met. Was it a chance encounter at a coffee shop or perhaps through mutual friends?
Love and self - discovery are often main themes. Through the marriage, the characters may learn about themselves and grow in their understanding of love.
Sure. 'My Happy Marriage' was adapted from a manga. The adaptation tries to stay true to the original source material while bringing the story to life on the screen.