You ask, why do some people stay in obviously terrible relationships and never break up?

Because they don't know the score, I guess.

Let's assume happiness, or what people often call "life is still bearable," is 10.

In an ideal situation, 5+5=10, and 3+7 also equals 10.

But many people forget that 10+0, and even 11-1, also equal 10.

We cannot earn money beyond our level of understanding, nor can we enjoy happiness beyond our level of understanding.

Maybe it's because everyone's expectation of love is addition; they haven't thought about subtraction.

Just like people habitually think that 10 is a perfect score for life, rather than just a passing grade.