I've observed that society is very judgmental when another person complains. It seems there is a lot of emphasis placed on whether or not a person has the "right" to complain based on the perceived level of difficulty of their life.
Personally, it has frequently rubbed me the wrong way when someone goes out of their way to call someone out on what they deem unnecessary or excessive complaining. From a perspective of motivation, I've always wondered what's really behind the act of doing so. I've always felt that the person doing this is actually just finding a subtle way to bolster themselves in front of everyone by implying that they are above complaining. My stance is: if someone is constantly making bad decisions and then bitching about their life as if they have no idea why they're getting bad results, then I can see calling them out on it. But aside from that, what is the motivation to police the complaining of others?
Also, is there a difference between "venting" and "complaining"? As I'm sure you've noticed, it seems that there's a perception that venting is more acceptable.