Student working retail declines to cover coworker's shift, leaving them wondering about whether or not they should be covering every shift they are requested to cover: 'They didn’t get mad, but they were clearly annoyed'

Working any type of role that relies on shift work, but especially in retail or in the service industry, relies on a delicate internal economy of exchanging shifts. 

You're never allowed to just take time off, and every time you try, your manager gives you a speech about how "Short-staffed" they are going to be and tells you that you need to "find coverage." But the schedule is never consistent or predictable, and your manager also "forgets" to take into account your scheduling preferences and restrictions, at any time that is convenient for them, and then demands that you figure it out for them

There will always, without fail, be that one coworker who always wants to give up shifts that are inconvenient for them, but is never willing to take on any extra shifts in return or repay any favors or inconveniences that they might have burdened onto their coworkers. 

It's usually either a veteran worker, too, who is older than their relatively young coworkers, whose life is somewhat in a constant state of disarray, and they seem to be wearing at the already threadbare ends of their own sanity, but can't quite see it themselves. For some reason, they've always gotten by by the skin of their teeth, with their immediate manager feeling that they have too much history together to simply let them go for reasons they might another employee.

Eventually, after going out of your way for them a number of times, only to have them bluntly refuse every time you have asked them to return the favor, you'll reach a point where you grow tired of sticking your neck out and covering for them. That, among one thousand other small cuts, will eventually lead to you reaching this point.

And, the moment you refuse to cover one shift, they get hostile and blow it up into a whole thing, and it expands into a "big deal" that involves the entire workplace. Just when it seems like things might be reaching a breaking point and someone actually is going to get fired, the manager steps in, calms everyone down, and sweeps the entire thing under the rug.  

You go on to watch them repeat this same pattern of behavior with every new hire that joins, and you start to realize that you, too, were really only one in the next in a long chain of fresh hires who will go through this same process. History repeats itself, ad infinitum, and likely will long after you leave that job.