즉석에서 그렇게 한거야.

A: I went singing with friends last night. It was fun.
B: Why didn’t you invite me? You know I’m into that!
A: 즉석에서 그렇게 한거야, otherwise I would have.
B: I guess I can let it slide this time, then.

A: 어젯밤에 친구들하고 노래방 갔었는데 재밌었다.
B: 나도 좀 부르지 그랬어? 내가 노래방 좋아하는 거 알잖아.
A: It was a spur of the moment thing, 안 그랬으면 당연히 불렀지.
B: 그렇담 이번에는 그냥 넘어가야겠네.

[Additional expressions]
*I went singing with friends last night.
– Last night, some friends of mine and I hit a singing room.
*It was fun.
– We had a good time. / It was a blast. / It was time well spent.
*Why didn’t you invite me?
– How come you didn’t let me know in advance? / Why wasn’t I there?
*You know I’m into that!
– You already know that I love to do that! / As you know, I enjoy singing!
*It was a spur of the moment thing
– It wasn’t planned in advance / It was an all-of-the-sudden thing
*otherwise I would have
– or else I would have invited you / if not, I would have let you know
*I guess I can let it slide this time
– I suppose I can forgive you this time around / this time, it’s “bygones