Blameshifting: What motivates it?

Our problems today, is partly caused by people's refusal to take responsibilities for their actions, as cowardly as this is, there are those who go to the extreme and blame others for their actions or its consequences in order to deflect guilt from themselves. My cousin had an encounter with a blameshifter recently.
My cousin, Alice, has always admired independent and strong women. From adolescence, she worked on herself to be able to achieve things without depending on others a lot. So when she was assigned a research project topic to work with a partner in the university as a final year student, she did not insist on his contribution to the demanding research study they were to carry out. Apparently, her partner is an undedicated student, who takes his studies very lightly, mostly absent from lectures. Being a kind lady, she advised him to come with her whenever she had to meet with the project supervisor so he would be well informed on whatever steps they were taking. But this was not to be for Mr. partner. Perhaps, he is a businessman, in school for any certificate whether it was one with a 3rd class honours on it or just someone who doesn't hold his education in high regard, therefore, it was of little importance that he participate wholly in the study. He decided to provide only the finance needed to print the project work despite Alice letting him know that their supervisor has a reputation for being too strict and that he requested he be present, most of the time. Alice consented to this, after all, "monkey no go do all the work make baboon come chop". Finally, the research work was complete and that's when her supervisor insisted that Mr. partner's name be taken off the project work if he was to sign on the project work,as far as the supervisor was concerned, the gentleman made no contributions to the progress of the research. Her partner went ballistic, called her names and blamed her for the supervisor's decision. In his words " As my partner, it is your duty to call me anytime you're to see the man". This is a student who seldomly attends classes and even missed some of his exams. Why do people blameshift? So that they can feel better or simply because they are cowards?