Why is the use of 니까 not correct in this sentence?

Question

I have come across the following sentences where the first one is correct and second is not correct

어제 아파서 못 왔어요. (O)
어제 아프니까 못 왔어요. (X)

I can understand that 아/어/여서 can't be used for imperative or asking someone to do something while 니까 can.

But I find it difficult to grasp the other difference between them, and the more I read the more I get confused about the "objective vs subjective" and "own feelings / situation". If 니까 is for "subjective reasoning", then suppose one was sick, actually some people may still choose to come, and some may choose to not come. So there is "subjective reasoning" here. Then why the second sentence is not correct ?

Answer 1

I think you're overthinking it.

Think of -니까 as something fluid, what you might still decide on or something that can change depending on your judgement (subjective).

-아/어서 is about more conclusive and unchangeable things, about which most people would think the same way. It is used for things that don't need much reasoning, like "We can't have the event because it will rain". Of course, some people might act differently in the same situation, but in that case, they can express that by phrasing it differently. So the difference between -니까 and -아/어서 is determined more by your own attitude about the situation than the situation itself.

Consider these:

1. 몸이 아파서 내일 파티에 못 가겠어.
2. 몸이 아픈데 그래도 너무나 가고 싶은 파티니까 가려고 해 (I'm still thinking of going because I really want to go)

#1 is like "There is A (being sick), so B (can't go) automatically follows, whereas in #2 the person is still thinking and deciding. You just pick the appropriate phrase that fits your situation.