Often, Goku's naivete about relationships in the original series is a starting point. In fanfiction, he gradually learns about love and the importance of Chichi. His kisses with her become a way to show his growth in understanding emotions. It could start with a clumsy first kiss and then develop into more passionate and loving ones as he matures in his feelings.
Well, in many fanfictions, their relationship often develops through shared experiences. For example, they might go through tough times together, like dealing with threats to their family or the Earth. These experiences bring them closer and make their kisses more meaningful, as they represent a deeper bond formed through adversity.
In a fanfiction, their relationship could develop as Goku becomes more aware of Chichi's needs. He might start to be more present at home, not just for training. For example, he could help Chichi with household chores in his own clumsy way, which would show his growth as a husband.
It could strain their relationship. Chichi might become more distant from Goku, feeling that he doesn't care enough about her safety. For example, if Goku hurts her during a fight on Earth that he brought to their home, she may start to question whether she can trust him to keep their family safe.
Their relationship could develop in a more complex way if Black Goku somehow gets trapped in a situation where he has to rely on Chichi. Maybe he loses his powers temporarily and Chichi has to take care of him. During this time, she might try to instill some of Goku's good values in him, and he might start to have a soft spot for her. This could lead to a very unusual relationship dynamic where Black Goku is torn between his evil self and the positive influence of Chichi.
It could strain their relationship. Goku might start to avoid ChiChi if he's mad at her. For instance, he could spend more time training alone in the mountains or with his friends rather than at home with ChiChi.
Highlight their shared experiences. They both have been through a lot with the Saiyan threats and other villains. Writers can recall those moments and show how they supported each other during those times. Also, their shared love for Gohan can be a great way to strengthen their bond. They both want the best for their son and can work together towards that goal.
One interesting scenario could be when Goku returns from a long and difficult battle. Chichi is overjoyed to see him alive and well. They share a tender kiss filled with all the emotions of relief, love, and longing that had built up during his absence.
In fanfiction, their relationship can develop through shared battles. For example, when they both face a common enemy stronger than either of them alone. They start to communicate and strategize, and in the process, they see each other in a new light.
Often, Kid Goku is portrayed as the carefree and adventurous type. He's always looking for the next exciting thing to do, whether it's chasing after a strange animal or exploring a new area. He might be a bit naive in some aspects, especially when it comes to things like emotions.
Miscommunication can also be a reason. If Chichi misunderstands Goku's intentions, like when he brings home a strange new power - up item that she thinks is dangerous for the family, and she scolds him without really listening to his explanation. Goku, who is not used to having to explain himself in detail, might snap in frustration.
Love and family are very common themes. Since they are husband and wife and have a son, many fanfictions explore how they deal with family life together.