The school year began.
I'm working part-time and need full-time.
I'm working a temp job and need a permanent one.
I've never really had a "bad" reason for leaving, like being let go, finding the work too hard, or having a co-worker conflict. Let's take the conflict with coworkers. To me, this is a perfectly logical reason for leaving a job - it can be really miserable to have to work with people you don't get along with every day. But my parents have always told me that this reason is a huge red flag to employers. It implies that you have trouble getting along with people and that you might cause trouble in the new workplace. They said when one person has a conflict with lots of other people, it usually means that that person just doesn't get along with anybody.
Okay, I'll buy that. That's ONE scenario. Now let's try another...
You're being sexually harassed at your current job, and you're afraid you'll get fired if you report it. So you seek employment elsewhere, and you keep getting asked why you're leaving your current position. You don't want to get into the details and have to relive everything during the interview. You're also wary that the interviewer might be a jerk and think that the harassment was your fault. So you say, "I had a problem with my boss" and leave it at that.
Or maybe you've been harassed because of your race, sexual orientation, gender identity, weight, or anything else that makes you different from the majority of your coworkers. Maybe a direct attack, or a series of microaggressions that no one even acknowledges when you point them out. Perhaps your work environment is designed with the assumption that everyone is the same in some way. While you really should be able to explain this problem to a future employer to assess whether the new workplace will be more accepting, you know that your interviewer might think you're complaining over nothing, or that it's asking too much for people to respect you the same as everyone else, so you just say, "I didn't get along with my coworkers."
Or maybe someone is just not being nice to you. A friend of mine once hired a person who had left their previous job because of a coworker conflict. Once they were hired, this person was really mean to everyone else in the office, and several people left because of the way this person treated them. My friend often sites this case as why you should be wary when someone gives "coworker conflict" as a reason for leaving. But when you look closer, you realize that for this one person who was mean to everyone, there were two or three other people who were very nice and left because they were being treated badly. Those people will also site the coworker conflict as their reason for leaving, just as the person who made them want to leave.
Or maybe you just don't get along with the people at your workplace, even if they seem to get along fine with each other. Maybe the environment just isn't right for you. That happens. When I think about the people I've had conflicts with, most of them are well-liked. When I think of the people that my friends and family members have had conflicts with, most of them are well-liked as well. We don't expect everyone to like the same kind of music or books, and we shouldn't expect people to like the same kinds of personalities or work environments.
I've always been told that when someone has a problem with everyone in a workplace, it usually means that that person is the problem. But I've been in that situation myself at college, and I will never go back to thinking that it must be someone's own problem if they have an issue with everyone else. The fact that I've never had a coworker conflict is not my own doing. Yes, I have been respectful to my coworkers and done my best work, but I have also never gotten the vibe in a workplace that people didn't want me there. And that's not job-skill-based; I'm sure a lot of that comes from being white, cisgender, straight, middle class, having the right clothes, and just being able to blend in with everyone else and do things without needing accommodations. Teachers and bosses have always gotten a positive vibe from me - they assume I'm reliable and trustworthy before they actually know anything about me. Unless I was at an interview or tryout, I've never had to prove myself to someone who didn't think I could succeed. Everyone isn't treated that way. A lot of people get initial suspicion from others instead of initial trust. And getting suspicious glares and evil vibes that people don't want you there is a perfectly logical reason for seeking a different job! If I were hiring someone who was leaving their current position, I would want to know why they were leaving. Not to decide if their reason was acceptable by some pretentious standards, but to know what went on at their last workplace and make sure they don't face the same issues when they're working for me.