This question is a bit misconstrued -- marriage for love was not completely unheard of, and because of this, there aren't any examples of 'first loving mariage'. Courtship was a done thing. Upper levels of society would make strategic marriages, yes, but women and men in lower levels of society had less property, wealth and influence for their marriage to be strictly political and economically-based. To better understand this, have a look at the sidebar for the FAQ about love and marriage.

In order to try and answer your question, I'm going to guess that you're referring to comparing historical standards of marriage to that of our current, Western society. I'd try to look at this as when did society begin to transform and when did new ideals begin to emerge on human rights because the more equal women are, the more agency they have to marry, the more marriages you see to 'mirror' the marriages we have had for the past few hundred years. You can read this review of women in the Enlightenment to better understand how the Enlightenment changed societal structure and ultimately resulted in our current way of life. (Obviously it's more complicated than that, but I'm not going to write out an explanation on the transformation of the early modern era, to the morality standards throughout the Victorian era, etc..)

Anyways, all in all, marriages for love have been common throughout history. I'd have a look at the differences between our current culture and outlook on marriage and gender equality and use that to draw conclusions, too, since your question to me really looks like, 'how did society change', as opposed to, 'how did marriage change'.