Prideful -- Incomprehensible -- unintelligent
Dogberry's motivation is pride. He thinks he's the best of the best and he should be respected by everyone, but, because he doesn't understand the meaning of some words, he makes himself out to be a really annoying and bad person. When he is interrogating the prisoners, Conrade calls him another word for "butt." This causes Dogberry to become very prideful and say, "Dost thou not suspect my place? Dost thou not suspect my years? O that he were here to write me down an ass. But masters, remember that I am an ass: though it be not written down, yet forget not that I am an ass."
Dogberry impacts the plot in a fairly big way. This is because he caught Conrad and Borachio talking about how he helped Don John frame Hero and stop her and Claudio from getting married. He arrested the two men and took them to the prison where they were interrogated. Don John, Conrad, and Borachio were all imprisoned for their crimes and Hero and Claudio were married.