Some people you just have no respect for. They might of just given you a horrid first impression, or they may have slowly lost your respect over the course of time. It doesn't matter either way, as no matter what they have shown themselves to be douchebags. Douchebags are all the people out there that really deserve no respect.
There are many breeds of douchebag. Much like with dogs the passage of time has allowed the douchebag to take many forms. Not all are easily identifiable, many have subtle tricks to hide their true nature. Some types are only found in certain places, or in a certain age range. Being a douchebag can be a temporary condition, but that doesn't make it any more acceptable. However, there are common traits to all douchebags that can help in identifying them.
There are a few common traits to all douchebags, and none of them are good things to find in a person. The first is self obsession. There is a belief deep inside the douchebag that tells them they are the center of the universe. The second is a need to impose their bullshit on to other people. A douche will go to great lengths to force other people to deal with whatever it is they are in to, or are taking part in. Thirdly, and this one is a bit situational, they will abuse even the smallest amount of power. Thankfully, not many douchebags have much in the way of power, so you won't see this trait expressed very often.
There exist far too many breeds of douchebag to name, at least here and now. There are some more common types you might run into. The boss douchebag, they are your boss, and they will ride that for all that it is worth. The internet douche, his opinion will be forced down your throat, if they are a mod or admin at a place you go just leave and never return. Workout douche, they has a lifting schedule, they either wants to impress you with it or tell you that you are in the way. Many more exist, and they are native to every place on earth.
Douches are those people that just don't deserve respect. You will have to deal with them from time to time, most likely on a daily basis. Perhaps someday a cure can be found for whatever it is that causes a person to become a douchebag.