Philosophy of love is younger than you might expect. Plato wrote about it in the Symposium — love as a ladder ascending from particular beautiful bodies to the Form of Beauty itself. But the systematic philosophy of romantic love really begins in the twentieth century, when the assumption that love sits outside rational analysis started to give way.
Harry Frankfurt argues love is a *volitional* state, not just an emotional one — to love someone is to be committed to caring about their good for their own sake, in a way that organizes your other commitments. This makes love continuous with what you do, not just what you feel. Robert Solomon (and the long tradition behind him) treated love as a kind of project — you and the other person create something together, and the relationship's quality reflects the quality of the creating.
Other traditions push back. Iris Murdoch, drawing on Plato and Buddhism, treated love as a particular kind of attention — the discipline of seeing the other person as they actually are, without your own ego distorting the picture. Care ethicists insist that love isn't reducible to either feeling or choice — it lives in patterns of responsive attention that come from being-in-relationship over time. The philosophical interest is partly diagnostic: what we think love IS shapes what we expect from it, and many modern unhappinesses come from importing one tradition's picture into a relationship built on another.