We know we need to pick our battles, but sometimes we feel like we need to pick some on behalf of others as well. While you might feel like you're supporting a co-worker, you'll probably just make yourself hate where you work.
Alison Green, writing for US News, explains:
It can be tempting to get involved in other people's grievances at work, but you can end up taking on the emotional burden of battles that aren't yours. For instance, if Joe hates your manager and complains about her all the time, you might find over time that you've come to dislike her too -- even though you got along with her perfectly well before. This can lead you to make bad decisions for yourself, like becoming unhappy with a job or manager you otherwise liked, or even leaving your job over it. This isn't to say that you shouldn't be sympathetic to co-workers' troubles or that you shouldn't speak up about serious workplace problems, but for routine complaints, keep in mind that you don't know the full story and try to stay out of it.
As we've learned from studies, venting your frustration only makes it worse. Don't compound that problem by venting frustration on behalf of your coworkers. Then you just end up with twice the unnecessary anger.
6 Ways to Be Happier at Work [US News]