From the word’s history
First used c. 1806.
大約在 1806 年首次出現。
One way to hold this phrase: picture something planted in the middle of your daily life, too heavy to move. You can walk around it, you can complain about it, but at the end of the day it is still standing there — the long queue, the rent, the weather your work depends on. That is a fact of life. The same phrase also gets used, in the plural and with the, for a subject adults find awkward to name out loud: sex and where babies come from. Those are facts of life too, and the wide, gentle wording is exactly what makes them easier to say.
可以想成一個畫面：生活正中央長了一樣東西，重到搬不動。你可以繞過它、抱怨它，但一天結束時它還立在那裡——排不完的隊、漲上去的房租、你吃飯要靠的那個天氣。這就是 a fact of life。同樣這幾個字，換成複數再加上 the，還會被拿來指一個大人不太好意思直說的題目：性，還有小孩是怎麼來的。那些也是生活本身的事實，而這個又大又溫和的說法，正好讓話比較好開口。