Both have a male lead, obsessed with cleanliness and with other PTSD and OCD problems displayed. He is a CEO, so he has to keep his troubles a secret. Is rich, but mistreated by his own family (which is the root of all of his psychological problems).
Female lead is poor, clumsy. Has problems hiding her own feeling as they are always being easily read on her face. He is business-smart.. she is innocent.

Both have a male lead with seamingly cold personality.. he searched for the person who saved his life (the female lead) for a long period of time (atleast a few years) and seemed like a perfect match for him, but it was deceived from another woman, whom he desired to date, but dumped after suspecting her sham. Then he meets the one for whom he searched for, but because of her strange behaviour suspects she is a paparazzi snooping for scoops about him or another gold-digger, running after him. Both male leads are obsessed with stuff being clean. The main heroine is messier than the male lead. Is sloppy and clumsy. Is poor, while he is rich.
It takes them both time to fall in love as the heroine have to overcome a long-time crush/love of hers.
The main hero chases after the heroine.. not quite realising why and whether he loves her or not.

Both dramas have glimpses of the relationships of two couples before and now.. and after their so-called breaks up (as in the Before we get married no matter how insistent one of the party was.. the other didn't want to accept a break up). They have cheating in it... The actual cheating with bodies. The cheating as in the emotional sense. In both one of the exes is exploring the other for money and is being nosy about his/her new relationship.

Both have a male lead who chases after the female lead even thought he is a few years younger than her.
Female leads are cutesy girls that slay with both beauty and skills.

Both have a female lead that is very professional in her work and a male lead that is very much younger that her.. is very much into her and know what he wants.

Both female leads were hurt by their past relationships.
Both have female leads who lost everything.. but within the show manage to build their team-mates and the relationship with their young partner.

In both couples there is one person whom is very much against marriage and one for whom that is a necessity at first, but starts considering abandoning that prejudice.

Both are school and family dramas which have a person who lived in small place in his entire life and a newcomer who appears his age and pretends to be a member of his family out of convenience.
At first the boys are very hostile one towards another, but later it grows into a strong bond.

Both gives us the feelings that they should have been a very light BLs, but tease us with a very strange bromance instead.

One of our leads is a dumb mermaid/merman arrives in the shore by mistake and the other one is the first person he/she encounters in there.

Both sea-creatures forms a strong bong with that person. They are about humans, mermans/mermaids and overall relationships and friendship.
Sometimes they touch the topic of how we pollute the ocean.

In both they are gready people who want to catch the merman/the mermaid for their own benefits.
In both they are glimpses of the merman/mermaid's past.

In both the merman/mermaid has a supernatural powers and strange urges. It shocks with unusual behaviour as it can't understand the human world well.

Both have a very influencial young male lead whom is mostly interested in his work. The female lead enters his world with an accident.

In both the male leads falls first and hard for the female leads and try to show their love with numerous of different gestures (like: meeting her with an umbrella in the hand while in the rain and etc.). They are openly expressing their jealousy.

Both male leads are very strong-willed and like to be dominant in their relationship with the female lead (even thought they are around the same age).
Female leads are very emotional and loving, but don't know when and how to stop their relationships with people.

Both have a cutesy and excentric female lead x strange and strict CEO. The male leads are only focused on their work and when they realise they are in love.. in the female lead.
The couple's are extremely cute when they are together..

In both just at the beginning of the series the female lead goes through a break off of a years long relationship and engagement due to her partner's cheating. At the same time she meets the male lead and makes an unusual impression on him.

Both females are very professional in their work, but very messy in their love life. They are lovely women who drag the relationship with their ex to the last episode.
Both males are very strict and sassy with both relationships and work. Sometimes a bit too strict and sassy.
Both male and female and co-workers who have admiration of the job of the other person.

Both males have a hard time believing in marriage due to having a hard time with their family and love relationship in the past.

In both the guy falls first and very hard for the girl. His love for her is very obvious to everyone around her (except her), because of his unusual dotting for her.
In both he turns out to be her boss at work.
In both she had a very long previous love-interest before meeting him and needed a time to move on.

The male lead in both is taken as "damaged goods" .
The female lead secretly replaces another woman in an arranged marrage-to-be and then marries the male lead, decieving him of her identity.
Later on both of them fall in love one with another.