Short answer
Many Koreans check saju before or during a relationship because comparing two charts (gunghap) is a long-standing family tradition, and today it is mostly a fun, social way to talk about compatibility.
The tradition
For generations, families compared a couple's charts before a marriage. Matchmakers used them, and a bad gunghap could give a nervous parent something to worry about. That is the version you see in K-dramas: the mother who quietly visits a fortune teller with two birth dates in her handbag.
How it works today
Today it is mostly social. Friends go to saju cafés, apps calculate charts in seconds, and people mention their type the way they mention MBTI. Some take it seriously, many take it lightly, and almost everyone knows their zodiac animal. New Year is a classic time to look at the year ahead.
Why it sticks
It gives people a language for differences. Saying 'you are Fire and I am Water, of course we argue about plans' is gentler than 'you are exhausting'. That is how we use it too: a way to talk about patterns, with jokes, and never as a verdict on a relationship.
Try it on your own love life
FAQ
Do Koreans really break up over saju?
Rarely, and it is not something we would recommend. For most people today, saju is a talking point, not a decision-maker.
What is a saju café?
A café where you can get a short saju or tarot reading with your drink. They are popular with friends and couples in Korean cities.
Which K-dramas feature saju?
Fortune tellers, gunghap and lucky dates appear in many romantic comedies and family dramas, often as a plot device rather than a plot.