A love story might be described as a narrative in which the love relationship (usually romantic) is the main driving force for the plot, and in which the major conflicts are strongly entwined with the livelihood of the couple.
While the romantic relationship between the protagonist Dana and her husband Kevin does have an impact on the characters and the plot, it is not the main driving force for the conflicts. The major conflicts Dana faces are more concerned with the relationship between Dana and Rufus, and with Dana and the past in general. While it could be said that Dana and Rufus love each other, it is more of a friends/confidantes relationship and wouldn't on its own be classified as a love story.
Although Kindred probably wouldn't be categorized as a love story, the romantic relationship between Dana and Kevin is an important part of the novel. It is Dana's love for Kevin that keeps her connected to the present when she is in the past (it is not her terrible job, nor her relationships...
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