Because we worked for the whole week on it and now want that other people can see it on their weekend. Since we are so proud of our work and trust in its quality, we are willing to do an extra shift on the weekend, should problems occur.
The experience which gave me perspective was on a project at my previous job. I worked together with another highly passionate engineer and we made amazing progress over the course of a very short time.
The day before the demonstration of our results to our bosses, meant to serve as evaluation and retrospective for this project, we had reached all goals and the simulation was really robust and stable. However, when I came to work on the day itself, I found my colleague sitting at his desk, looking very tired. He'd been working through the night to get all the extra bells and whistles implemented. The demonstration was a disaster. Nothing worked, everything crashed. A lot of people were upset.