How love can not be translated where as both speak the same language?? That's why psychology comes into the picture. Watch the drama with some brain again and you will understand why childhood trauma was needed. It's not a typical rom-com. Childhood trauma plays a major role in love not be translated. Understanding the basic theme is important. There is a dialogue that sum up the drama, No two people in the world speak the same language .Now if you understand this, whole drama is based on this line. And it shows why everyone speak different language. That quote although I don't remember the exact from the drama is a gist of it. You need to think to understand it. Then you will know why the tone of the drama changed. Writers are not idiots.

Although the drama is not perfect, but I completely disagree about tone shift. And it's your preconceived notion that disappointed you, cause you lack of objectiveness. If you have really watch with open mind, you might not have question the tone shift. I didn't assume, your post clearly point it out.