What is the theme of "The Lottery?"
One of the main themes in "The Lottery" is the cult of tradition. People do things out of habit on a daily and even yearly basis. Habits become rituals which can become traditions. When social institutions (such as a church, government, or company) establish traditions, they become rituals. After so much time has passed, people find it difficult to part with tradition, no matter what it is.
There are whispers in the crowd from people who've heard that other towns are no longer doing the lottery. Of course, the oldest villager, Old Man Warner, the one most steeped in tradition (and a stubborn lack of common sense and morality), scoffs at the idea of no lottery.
"Listening to the young folks, nothing's good enough for them. Next thing you know, they'll be wanting to go back to living in caves, nobody work any more, live that way for a while. Used to be a saying about "Lottery in June, corn be heavy soon.' First thing you know, we'd all be eating stewed chickweed and acorns. There's always been a lottery," he added petulantly.
Old Man Warner never gives a reason why the lottery is performed. He just says it's always been. In this case, a tradition such as this ritual is not performed to honor a worthwhile historical event or some cultural figure. For these people, the ritual is done for its own sake. It is a mindless adherence to a ritual and it is a cult of tradition.
Some traditions should be celebrated. But traditions and rituals which do harm should obviously be dropped. This seems like an obvious point to make, but there are traditions and/or traditional beliefs today that some people think are harmful. For example, some people think that the "traditional" notion of marriage is between a man and a woman. However, many people believe that this tradition needs to change to include same sex marriages. Those who oppose same sex marriage use religious doctrine and procreation as their arguments, but they also tend to use the empty argument: "That's the way it's always been."
Here is another example. Some people think that the Confederate flag is offensive, but many southerners feel that it is part of their tradition.