So many of our conflicts and frustrations come from disappointments with other people. They didn’t do what we had hoped or expected. They came later than they said they would. They didn’t give a good enough compliment. They forgot your birthday. Again.
The best way to avoid being disappointed is to accept people for exactly who they are.
If there is a pattern of forgetting your birthday (“How could you forget my birthday? Every. Single. Year!”), you could be disappointed (again!). Or, you could accept that’s who they are. They may need reminders. Or just accept that your birthday will never be celebrated in the exact way you dream.
When you accept them as they are and drop the expectation, you will avoid being disappointed. Even more, it probably won’t bother you at all.
To really accept them, don’t even try to figure out the psychological reasons for them being who they are. Just accept. Notice and highlight all their wonderful aspects. Love and appreciate them as they are.