Forgive me for being naive, but I’m curious as to when the modern view of marriage being about love and relationship began becoming the mainstream purpose. Or has that typically been the predominant reason, with the trading aspect being solely oriented to specific cultures or nobles/royalty?
Although there were often economic incentives to marry, love was also considered an important motivation for marriage in various cultures.
Here are a few detailed and well-sourced answers from the FAQ on Love & Marriage:
First and foremost is this excellent thread on the context of marriage in the Western World before Christianity with contributions from /u/sunagainstgold, /u/yodatsracist, and /u/XenophonTheAthenian
How did the concept of marriage originate? by a now deleted user
What was marriage like between Swedish peasants? by /u/vonadler
What was marriage like in ancient Egypt? by /u/cleopatra_philopater
In Medieval Europe, if a female serf was very pretty would the Lord who owns her land simply take her as his wife? Even if she was already married? by /u/sunagainstgold. Her answer on the average age of marriage in Medieval Europe here and here also discuss the economic motivations to marry.
This isn't to discourage any new answers or debates. In fact, since attitudes towards marriage vary so much between cultures and time periods, there's always room for additional answers.