Too bad for us that the underlying belief system is a basic premise of the strip. Every so often, we get a reminder that Elly believes herself to be far too old. Why, I can see her fear of being decrepit helping to make marrying a nerdy dentist a good idea. If you can't see her sitting in her dorm thinking "If I don't marry at the ripe oooooooooooold age of twenty two, I might as well give up and become an old maid who'll never have the family that Society expects of me." Given this belief, it's not much of a surprise to find out that John and Elly hate celebrating their birthdays owing to their annoying and silly belief that by marking such rites of passage, they're actually accepting their imminent death. Given that their friends, neighbours and relatives don't see them as being within milliseconds of dying of Super-Crazy No-Way Stroke Cancer, what happens is that we get a panicky idiot whining about a non-event that their friends see as a good thing.
That being said, we also have to contend with the anti-fun philosophy that guides Elly. As I'll remind you tomorrow, we have to remember that John and Elly think that they're doing a good thing by trying to destroy their children's ability to enjoy life. This, I think, is owing to their belief that by making them far too serious for their own good, they're improving them morally. This means that Elly fears and hates children's birthday parties because it might make the kids think that just maybe, pleasure isn't an evil to be eradicated.