It happens all of the time. You do an event, project, host a party and someone right after or during wants to talk about who isn't there. Who didn't show up, why they didn't, and analyze the reasons they weren't there. Basically, discounting everything that you have worked towards and focusing on who wasn't around to witness it.
Human nature? Maybe, but I think we need to put a stop to it. We need to take a moment and celebrate the good that we are doing. Not in a PollyAnna kind of way, but in a refreshing look at the momentum that was created to get from Point A to Point B. Talk about what is going right, who is there, how the project or party made a difference. We spend so much time tearing what we do apart that we can't even enjoy the good that is happening right around us.
There will always be time for careful reflection about how to improve the project, but even the negative can't overwhelm the positive. Sure, you might not do it the same way next time, but you can take in some good vibes for what you did do.
And what if someone is always critiquing who is there and who isn't? Shut them down. Seriously. Tell them that you need them to focus on what is happening and attend to whatever needs doing right then. If you let them continue down the line with their negative talk you will soon be on the "my event sucked train". Seriously. So, shut them down and start enjoying the good you are doing.