So this might be very far fetched in terms of similarity but I did and do get the same feeling when watching both dramas. Both revolve around a small group of friends that have known each other for many years and that were involved with one another at some point. Things happen and they grow apart and things happen that make them come back together and try to work through it. There are many different storylines that somehow are all interlinked. Some will make you angry and some will make you feel sad. There is a lot of healing needed and a lot of hurt to go through.
And both dramas are brutally realistic.

Apocalyptic future, a secret organization that tries to get a person that has special and unique abilities. A female detective that gets pulled into this organization by chance. Mysteries annnnd a very similar ending.

Both dramas are part of the Light on Series and both are dealing around a crime case that is similar to one that happened more than 20 years ago. In both cases the family of the victim try to find out what has happened with the current case as well as with the cold case from 20 years ago.

Basically the same story. Both dramas deal about fathers that lose their teenage daughters. In both dramas the daughters had absolutely unthinkable things happening to them. In both dramas the fathers are out for revenge.

Both leads grew up together, are best friends and end up living together. Both couples have feelings for one another. The only big difference is that in I need Romance 2 the couple already dated before.

Best friends grew up together, their professions are helping one another and slowly they start to realize that they mean more to one another.

Best friends, that grow up together realize as an adult that they mean more to one another. In both dramas the male lead realizes his feelings first and the female lead slowly realizes them too. Cute dramas about a similar topic.

Best friends to lover trope. Something happens and they have to sort of live together and slowly feelings get revealed and confessions made.

Best friends, that grew up together, live/start to live together and that feel more for one another. Both stories are quite similar, the only big difference is that while More than blue makes you get out a big box of tissues, Soundtrack draws a big smile on your face

A pair that has an instant connection. A hate relationship that turns to romance. One (Liar) or both (Before we get married) that are in relationships and face infidelity, while being engaged to someone else. And fiancé(s) that cannot let go.
Both dramas are nearly the absolute same story.

Both dramas are about a crime that is connected to someone on the police force.
In both dramas the officers know the suspected culprit well, are estranged from them and are now involved in their cases. Both keep secrets from their colleagues and both dramas have the subject of poverty, abuse, suicide and bullying.

Extreme bullying and victims taking their revenge in a specific way that is in both dramas similar. Both dramas also have the theme of mentally unstable people and both dramas are filled with twists and surprises.