This study aimed to reveal the risk factors for a person to perpetrate stalking-like behaviors following the end of a romantic relationship based on personality traits (attachment anxiety and narcissism), the characteristics of a romantic relationship before a breakup, and the emotions and thoughts of a person after a breakup. To develop two scales measuring the characteristics of a romantic relationship before a breakup and the emotions and thoughts of a person after a breakup, a web-based survey of 189 females and 165 males was conducted in Study 1. In Study 2, a national survey was conducted using two-stage stratified sampling; 106 females and 110 males who experienced the end of a romantic relationship during the past five years and did not initiate their most recent breakup were analyzed. The results of a multiple-group analysis revealed that both attachment anxiety and feelings that a partner was his/her “one and only” increased egoistic preoccupations after a breakup, and the egoistic preoccupations predicted the perpetration of stalking-like behaviors in both males and females.