Young Hearts: The Courier has to tell Janet that Jack is in love with her. After telling her she says she has the same feelings for him, but can't leave because she's under contract with the Crimson Caravan and can't leave without forfeiting her pay.
PC After completing Cass' quest Heartache by the Number with the violent solution (unless you Sneak kill Alice) or if the Courier's reputation with the NCR is at Vilified, Crimson Caravaneers become hostile and Janet is included in this. To bypass, use NCR affiliated armor and they should ignore you enough to complete the quest. [verified]