Because it's such a pity if you start off on a bright tone on a story and suddenly realize you are talking about some people having been killed in a road crash.

因为如果你一开始用欢快的语调讲一个故事，然后突然意识到你讲的是一些在车祸中丧生的人，这是很遗憾的。

The miseries I went through made me suddenly realize with a blinding flash what life was all about.

我所经受的苦难令我豁然顿悟生命的意义。

I suddenly realize it's already 7:00 a.m.!

我突然意识到已经7点了！