My previous analyses of love have emphasized tenderness as care for the other's vulnerability as revealed in the originary event. We cannot protect against natural mortality, but we can defer violence toward each other.
Tenderness is a gesture to one mortal from another only inconsequentially less so.
The common reciprocal use of this expression marks the integration of the romantic love of the troubadours into the bourgeois family. Love is not always requited. The declaration solicits a symmetry, but in a necessarily asymmetrical manner. This is the stuff of romance narratives. Fearing rejection, one waits for confirming indices, or for a declaration from the other.