It's a tradition bound up with a lot of dodgy history - these days (hopefully) usually entered because of love, but historically more to do with property/money rights, political/social power, shame (eg preggers)... some cultures the parties haven't even met before the wedding.
My wife and I are married, reasonably happily - but I do wonder what the future will look like. "Thruples" and polyamory is becoming more widespread as the cultural imperative to have the same partner for life reduces. When you think of it, love and attraction are not limited to one person (even at a time) - so if shame and jealousy were removed then all sorts of combinations are possible.