Arguing in a dream often surfaces a tension that hasn't found its way into a real conversation yet -- feelings you've suppressed rather than voiced, playing out where it feels safer to let them show. The person you're arguing with may simply be standing in for whatever the actual disagreement is really about.
The aspects of the sextile or trine have the same quality: but the sextile is less forcible than the trine, either in good or evil. - William Lilly (1602.-1681.)