Well, birthdays are merely symbolic of how another year has gone by and how little we've grown. No matter how desperate we are that someday a better self will emerge, with each flicker of the candles on the cake, we know it's not to be, that for the rest of our sad, wretched, pathetic lives, this is who we are to the bitter end. Inevitably, irrevocably; happy birthday? No such thing.
Jerry goes out of his way to NOT be funny with George's girlfriend.