Why is the onus on us?

Could you please elaborate on why the responsibility or burden, as mentioned in the phrase "the onus on us," is placed upon us? Are there specific reasons or circumstances that have led to this conclusion? Are there any external factors or decisions made by others that have contributed to this sense of responsibility? Furthermore, what are the implications or consequences of this responsibility being placed on us, and how should we approach it in order to ensure a positive outcome?