When lagna or ascendant of both the person happens to be same or moon sign happens to be same there is possibilities of affinity. 5th house indicate affair , 7th indicate marriage and lagna lord indicate will power or persistent desire to marry. Combination of 5th 7th lord aspected by lagna lord or 9th lord indicates love marriage. There are other combinations too like interchange of rasi of mars verses venus and both the 7th lord aspecting each other. Like wise malefic in 2nd or 8th house indicates hurdles in love marriage.