Medieval covers 1,000 years, so I would not dare to venture an opinion over all of it. However, for the pre-industrial western world in general , you can say that marriage was usually far too important a matter to be governed by love. Most of the economy was based on land: farming, certainly, but also things like mining coal and managing a woodlot for timber and firewood. Land , as they say, isn't being increased ( yes, eventually there are colonies, and then that changes ) and so to get more land, someone could inherit it, conquer it, or be given it by the monarch- but those could be rather difficult to actually achieve. Someone would have to be rather lucky. Marrying it, on the other hand, was often far more possible.

But if you imagine a village circa 1600 in , say, Devon, those possibilities could still be rather few. There's just so much land available. Can another farmer fit in? Likewise, for trades. There's only so much trade for a craftsman, and tools are expensive...the village has perhaps one blacksmith shop. Is there room for another blacksmith shop? Perhaps not. A marriage would add a new element to the community, a new pair of adults who were the head of a household and , presumably, having children, and needing to live. Was there a slot open for them to fit into? Was a tumble-down building there for restoration, was the blacksmith getting too old to strike the iron and his journeyman getting on well? Had the farmer's second daughter , serving for several years at the manor house, collected enough of a dowry and retained enough vails ( small gifts given by guests) to be able to think of marrying?

So, the impression you get of marriage in this world is that it was constrained. If the blacksmith wasn't too old for his journeyman to take over, that journeyman would wait to get married. Or, perhaps more precisely, he would be made to wait. Because marriage involved the parents- the father and mother of the possible bride would typically be approached by the father and mother of the possible groom to see if he might visit. If there was any kind of excused denial- ( the pre-industrial equivalent of sorry, she's washing her hair that night) - it was taken as a refusal and that was that. If a visit was possible, matters might proceed- but only up to a point. Because although there were many practical constraints, there was also the fact that the possible groom or bride could, on further ( and supervised) acquaintance , say no. Or the girl could say no at the outset. The girl could be told she had a good man, that it was a good match, good for the family fortunes, etc. etc., and she might well look around and decide she was never going to do any better. But, in the end, if she didn't want the guy, it didn't happen. This last applied to all sorts of people, by the way, even aristocrats. When the future Charles I rashly sailed to Spain to woo the Infanta, it would have been very advantageous to the Spanish in many ways to encourage the match- they were at odds with France. The Infanta, however, was horrified at the prospect of marrying a heretic, and her opinion in the matter was important. ( There were many other problems with the marriage as well, but that was a big one).

Now, these were humans, and so all sorts of things did happen that weren't according to custom and plan. Girls eloped with the wrong man, or got pregnant before they were married and were abandoned, boys left the village to try their luck somewhere else. This is the source material for fairy tales of buried treasure revealed, clever Jack venturing out and making good, or the sad ballads of the girl murdered by her evil lover after revealing she's pregnant. But the question is about marriage.

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When did all this stop? Simple answer is, when wealth stopped being tied to land, could actually be created, after the industrial revolution. When the farm could be left for the city, when a job might lead to a better one. But even then: what we think of as the strict rules of Victorian society actually went much further back. When Grover Cleveland, in middle age, became interested in a younger woman named Frances Folsom, he first wrote to her parents asking if he might write to her. Any villager of the 1600's would have thought that quite right.

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EDIT: A very good elaboration of all this is in a classic, Peter Lazlett's The World We Have Lost