This is not a guide to remember usernames & passwords.
I’m just giving the reasons why we usually forget our login credentials. This is from my perspective, so it may differ from the actual research if there is any.
1) you have many username and password combinations you can’t remember.
2) you use remember my password facility in the browser and in other applications.
Though it reduces strain to enter login credentials but there is a chance that you may forget them, because you never enter them manually. which means the instance of password is not refreshed on your memory/brain.
3) creating a new account whenever you forget the old one.
4) using your account very rare
5) carelessness & feeling of unimportant
If there are any you are free to add them.
Thanks for reading please comment your feedback.