Before marriage, matchmaking of bride and groom’s horoscopes is very important.
Only Gun Milaan (36 guna matching) obtained by matching of nadi, ganna, bhakut, grah maitri, tara, varn of the boy & girl is not sufficient for a happy married life. I see many more things under matchmaking like navmansh chart, mangalik dosha, longevity, character, luck, health, wealth, family and children yogas in both the boy and girl's horoscopes.
There can be many types of malefic yogas present in the horoscopes like short life, no child, divorce yog, manglik dosh, sadesati, kalsarp dosha, pitru dosh,extra marital affairs which can break a marriage even after a good score obtained in Gun milaan chart.
So a thorough matchmaking before fixing a marriage is very necessary for a long lasting & happy married life.