Basically the point of good news, bad news is to post something that either is happening in real life or made up that is good news. However, the following poster adds a line of bad news that goes along that supposedly offsets the good news.
Example:
I just won $100 on a lottery ticket.
The wind blew it out of your hand on the way to the store to cash it.
So lets give it a go.
I'm glad its bright and sunny outside right now.
