They must act as if they actually like one another. Everybody makes a fuss about you, starting with your parents. Just the day before, they may have been grousing about your quarter grades, the slow response time on chores or the clear, unreadable look in your eyes that scares them from time to time. In truth, they were ready to throw you out. On your birthday, all judgments are off.
Birthdays are a way back to childhood.
We photograph, we paste pictures in their childhood books, we remember. Maybe you're not cute like that anymore, but on your birthday, parents can fudge with impunity. They can toast the early days when everything was simple except trying to figure out how they were going to get more sleep. The times in the hospital when they were close and the world couldn't touch them. They toast you and for a moment you're almost perfect.