You left your boyfriend at your apartment. He let the manager in. Why is it the manager's fault? Your boyfriend is the one who owes you loyalty.
It might be human nature to blame the "other" woman/man?
To blame your partner is, in a way, also blaming yourself (e.g., there is something wrong with
you if your partner cheats;
you are a poor judge of character if your partner cheats ...).
It is so much easier on the ego to blame the other person.