It's expected that you'll help your coworkers.
Sammy's coworker asks Sammy to stay later with her to finish a big presentation.
Sammy wants to go to dinner with her boyfriend.
Sammy stays back and helps the coworker with the presentation.
Sammy blows off the coworker to go to dinner with her boyfriend.
The presentation goes great and the company has enough money to give Sammy her promotion.
Sammy's coworker does terribly on the big presentation and Sammy misses out on a promotion due to budget cuts.