Grahmaitri is very important because it indicated friendship between the Rashi Lords of both horoscopes. A bad Grahmaitri indicates two individual who are totally strong headed and uncompromising towards each other. They tend to make issues personal and take it to the level of ego. Uncompromising behaviour can become most important reason for marital discord. Say for example Saturn and Sun, or
You can take a detailed horoscope matching which will discuss both charts marriage in depth along with detailed analysis of Guna milan and Grah Milan which is very important. Many people ignore Grah milan which is more important that Guna milan. Grah milan is matching of all planets in both the horoscopes. You cn read more about Guna and Grah milan from the link below.