If you're just starting out, or even an intermediate, you might be wondering why some people might get chosen for jobs over you even though you might have more experience. There could be lots of reason (and often it has to do with familiarity with the person that is ultimately chosen), but Adam Everett Miller, the editor for Aetuts+, sits down and goes over 5 reasons why a professional might be reluctant to want to work with you -- or in his words, "5 Things That Tell Me You're Not a Professional."
I changed his own title a little bit, because I think if you're a professional and you do any of these things, you're either stratospherically talented, or you've paid your dues after a long time. Neither of those are excuses, but Adam's title does suggests unprofessional behavior, of which anyone can be guilty of. I've watched through the entire video, but rather than listing the 5 reasons, if this video applies to you (it could really apply to anyone), I think it's important to watch all 15 minutes as it goes more in-depth than a simple list ever could. I will, however, choose the reason I actually think is the most important to avoid, and that just so happens to be the first one: negativity.
Filmmaking is all about problem-solving -- in fact, anything that can possibly go wrong usually does. The hardest type of person to be around during those incredibly difficult times is someone who only sees the negative in every situation. I've been on my fair share of late night through early morning shoots (I've caused a few myself), and they don't usually go very well when there are individuals who complain about everything. Often, that negativity is like poison that finds its way into every single crew member if given enough time. It's draining, and it makes the entire situation that much more difficult. If you find yourself working with those people once, I guarantee you're going to think twice about wanting to work with them again. If you're in a position to hire, that person might very well be at the bottom of the list.
Now, that's not to say that all negativity is bad -- but as Adam says, there is a right way to do it. On a film set, the entire crew is in it together, and each individual piece is important to the whole. It's human nature to be unhappy at times or to have creative differences, but those people who you hire again or work with again are those that are able to quickly find a way to work through their own issues and continue working hard for the good of the project. You're going to work with people who are inexperienced or incompetent at some point in your career, but how you deal with that says more to do with you than it does about them.
Here's something I think is important having written for this site for a while (especially for younger readers): your online persona (whether anonymous or not) carries a tremendous amount of weight, and it's very telling about the type of crew member you'll be simply by how you treat others online. If you're respectful, there's a good chance you will also be that way in real life, but if you're constantly combative and negative, it's not hard to reason that your behavior at some point on set will reflect that.