Not always always, though yeah... rather frequently. But how would you feel about a giant creature that can just walk all over dozens of knights, raid a few towns for food he could have just bought, treasure he's going to sleep on, or just for fun? Western (Europe) dragons are not very nice, generally speaking.
Eastern dragons are more often the sagely type, so you might wanna check on them.
You just answered the question of "why are dragons always portrayed as bad things" by simply making a list of negative portrayals and asking how he feels about them. Bad answer. Anyway, to actually answer olafanderson: because the dragon's basic and essential characteristics remind us of negative aspects. Its horns resemble the horns of demons and the devil, its body is serpentine like a snake, its fire is like the flames of Hell, and its passion for gold represents greed. Also, the stereotypes enforced by powerful religious beliefs hold the eastern dragon's reputation in its place, considering the fact that some will believe that said stereotypes are actual truths rather than just fairy-tales. And lastly, dragons are impressive foes, so many cast the dragon as an obstacle that must be overcome. Killing them can enhance a hero's valor or just make an impressive climax, but also the audience must want the dragon to die in those situations, so it's made to be evil.