It comes like that. With a total stranger
We fall in love so that we can form lasting bonds which help nourish and protect us, especially at vulnerable times such as pregnancy and child rearing, we were originally designed to have both parents present and contributing to the rearing of kids ans so love and close bonds help make that possible..
Of course, here in the real world that doesn't always happen and people can and do end up raising children alone.. but I think most would still say that isn't ideal..
generally speaking while we can fall in love at first sight, it doesn't have to be with a stranger.. it is most often something that develops with someone we know