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 cannot leave because she is under contract with the Crimson Caravan and cannot leave without forfeiting her pay.
After completing Cass' quest Heartache by the Number with the violent solution (unless the Courier stealthily kills Alice), the Crimson Caravaneers become hostile, including Janet. To bypass this, use NCR affiliated armor to complete the quest. [verified]