Despite his aloofness and cynical sense of humor, Mulder displays unbridled enthusiasm and interest when it comes to the paranormal, especially because of his personal involvement after his sister's abduction. Walter Skinner and other FBI officials have become concerned by his unprofessional behavior. This only proves true as Mulder makes a habit of letting his personal feelings cloud his judgment; his emotional attachment to UFO cases becomes more severe throughout the series, because of Dana Scully's eventual abduction and resultant cancer as well as his sister's abduction. Mulder can lose his temper when Scully is involved; on multiple occasions, he has become violent in his grief and unreserved in threatening physical force. Mulder's overprotectiveness of Scully stems not only from his obvious devotion and love for her, but also a long-harbored guilt; Mulder has admitted feeling indirectly responsible for the ordeals and tragedies Scully had gone through because of their investigations.