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