Mars is about action, whereas Mercury is about mental activity. An action expresses itself better when it is physical. On the other hand a mental activity can be done better sitting quietly not doing much of physical movement. So, Mars and Mercury have opposite nature ……

For more information on this, the exclusive content at http://courses.manishverma.site/mod/glossary/showentry.php?eid=215 and also about Mangal dosha can be referred.