I wanted to watch Happiness originally when it came out but it took some time before I actually got to it. And to be honest I don't regret it. This is a drama for binge-watching for sure. And before I go more into why, let me tell you, this review says 8.5 but my personal rating is 10. This drama is definitely not for everyone, a lot of people might even drop it. But let me tell you more...

What is old/what is new: The story sounds like your typical zombie post-virus infection story with people turning out to be the real monsters and turning against each other. There are a lot of those dramas and movies going around, in that regard the drama is very cliche. What makes the drama stand out is mainly the characters, relationships and the disease itself.

Unusual relationships: The main leads are in quite the unusual relationship and I do have to say I absolutely loved that they weren't your usual lovey-dovey couple who have to face crisis or bickering strangers who become each other's savior. First and foremost they are friends, they trust each other and they even trust in each other's abilities. And at the same time they care for each other in a loving and pure way. It's just such a fresh breeze compared to all the overly dramatic love story lines.
There are many other interesting relationships and characters in the drama, some expected, some unexpected and despite the obvious "villains" you will end up liking some of the characters you might have disliked in the beginning. There is just no chance to hate them and their character development is astonishing.

Character consistency: One thing I absolutely hate in dramas is if characters aren't consistent. If it's part of their character development and their change makes sense, then fine, but if it's just a 180º from their usual actions, it's just unnatural. Happiness does a good job at this. There is character development but at the same time all characters are consistent in what they want and what is most important to them. You get to understand their motivations and how they think and you understand how they arrive where they are at the end of the journey.

The disease: One of the most interesting aspects of this is the disease itself. Not like other zombie dramas, the focus of the story is not just on surviving but also healing. The characters never really forget that there might be a cure out there. And if there is a cure, is it really reasonable to kill a person who is infected? The cure is definitely a key focus and plays a big role.

The moral dilemma: For me there are two big questions in this drama, one being the typical "What actions are reasonable when someone is trying to survive in an apocalypse?" and the other unusual one is "Does my happiness always mean making someone else miserable?" Everyone in the drama tries to achieve their own happiness, in whichever way possible, but at the same time some find it in themselves to overcome their own selfishness. I love how the leads never really make it a "will we act in everyone's best interest" or "will we act in our best interest" but they clearly thought their and the other people's happiness didn't have to exclude one another. An interesting take on humanity and happiness...

Watch if...
you like romantic (as in it touches your heart) but not too lovey-dovey couples, zombie shows, mystery solving shows, fast-paced shows.
Do not watch if...
you love romantic dramas (or rom-coms), are easily scared of zombies and blood or if you expect this to be an absolutely mind-blowing drama. If you don't like it after 2 episodes, don't continue watching.

Is it a must-watch? No. Is it worth the time watching? Yes.
This drama is definitely a nice break from all the usual dramas, shining with their character design and acting (which honestly is top-notch on all sides) and gives you quality drama time. I hope you enjoy!