Proctor feels that Parris is selfishly abusing the power of his position in Salem. Parris complains that his salary is too small and insists that he be given firewood. Proctor feels that Parris has no regard for God and thinks only of himself and of money.
Early in the play the differences between the men are energetically debated. At one point Parris criticizes Proctor for not attending church. Proctor responds by outlining the various abuses he feels Parris is making through his position as reverend.
"...the last meeting I were at you spoke so long on deeds and mortgages I thought it was an auction."
Parris then expresses his belief that there is a group setting up against him (Parris) and that Proctor is part of that group. This is not true, but in anger Proctor says that if such a group exists he will join it.
The basic problem Parris has with Proctor stems from Parris' understanding that his position in Salem is not secure. This is a result of his own behavior. He will not admit this much, but is quick to address the issue of his own insecurity in Salem and to defend that position in whatever ways he feels he can. This includes attacking Proctor (a man who openly challenges him) and supporting the witchtrials (to deflect attention from the fact that his daughter was caught dancing naked in the woods at midnight).