It seems like the idea that one would get married solely for love is actually a fairly new concept. Am I in the wrong for theinking that, or is that not the case?
This is very broad question-- I'm going to assume you're talking about Western Europe, specifically England and France and maybe Germany and Italy to a lesser extent (because that's where my expertise it).
I'm pulling mostly from Lawrence Stone's Family, Sex and Marriage in England in the History of Privacy, volume 3, by Aries.
Prior to the sixteenth (and early seventhenth century), the commonly-held principle was that personally-selected marriage based on love, sexual attraction was far less likely to produce lasting happiness or stable relationships that produced lots of children--very important in primogenital households where male heirs inherited the family estate and fortunes. Indeed, the tragedy of Romeo and Juliet to a person in Shakespeare's time wasn't their romance and intense love, it was the manner in which they destroyed themselves and their families by violating societal norms: "An Elizabethan courtier would be familiar enough with the bewitching passion of love to feel some sympathy with the young couple, but he would see clearly enough where duty lay." Therefore, marriage was arranged by the family and a council of elders, which could include aunts, uncles, the family priest, close friends, etc. The decision was often made based on the potentials spouse's wealth, heritage, power, or family connections.
For Lawrence Stone, it is protestant moral theology that started the shift, for Aries et. al, it was more a development of the regulation of adultery and immorality which was being prosecuted in secular courts, but either way, in the late seventeenth century and the early eighteenth (1680-1730) the attempt to reduce adultery gave to children, for the first time, the right of some say in their spouse, in the form of a veto. "This right of veto could only be used with caution and probably only once, or at most twice, while for women there was always the risk that its exercise might condemn them to spinsterhood, if their parents failed to provide another suitor."
So, to get to your question, in the late eighteenth century and the early nineteenth (1780-1800+), this veto eventually developed into the child's right to choose their own partner. Stone argues there are three reasons for this:
The first is that the nuclear family has become relatively independent of the kin, so that decisions about marriage are no longer made by a grand family council of elders, whose main concern is necessarily to defend and advance the interests of the clan rather than to gratify the personal wishes of the individuals, although usually they will genuinely seek to find a compatible mate.
The second is that close parent-child bonding has developed, so that parents are reasonably satisfied that their own values have been internalized in their children and the latter will therefore make their selection from within the socially appropriate group.
Thirdly, parents must be willing to grant adolescents considerable freedom to meet on neutral ground with members of the other sex and to develop their own courting rituals of conversation, dancing, etc.