I see a casual acquaintence down the hall. "Hey, how are you?" I ask. "Bad."
I'm startled. I expected the answer to be "Good, how are you?" Instead I'm thrown head first into a conversation of his bad day without being able to change the topic from what I'd been focusing on before. My friend was impolite. I really don't mind listening to him talk about his day, but a little warning would've been nice. If I had said "How are you?" and he said "I'm alright, how about you?" or "Been better, you?" well then. I can say that I'm fine and ask what's wrong and now we've comfortably slipped into a conversation about a bad day. In short, these responses are more polite. In short, polite speech is intended to be just that: polite. Nice, comfortable and friendly.
In the teenage world, many formalities are ignored. We don't dress fancy, we're not exactly quiet, and we're not too big on shaking hands with each other. But, polite speech is something we hold on to. We say things like "Nice to meet you" when we're introduced to someone our age by a friend. "We should hang out one day" is also a big one for my age group. Do we mean it? Not always. Why do we say it? Consider the other option.
"Person A, meet Person B"
"Hi Person B, I didn't want to meet you."
Well they were honest. No, not honest, they were rude. Who cares if they didn't want to meet Person B? Who cares if Person B looks like a jerk and kind of smells bad? Person A ruined that whole exchange, not to mention they embarassed Person C who introduced them. Long story short, now they're all pretty uncomfortable. No coming back from that one.
These polite terms are probably evolutions of older greetings and goodbyes. When people say "good day" was it always a good day? Probably not. When saying "charmed" after meeting someone, were people always charmed? I'm going to assume not. Our language has evolved to the point where "What's up" can replace hello and "It was good to see you" can replace goodbye. Rejecting them rejects our current social constructions. Honestly, some of our modern constructions should be rejected; they're outdated and bigoted, but these polite phrases shouldn't. They serve only to foster kindness.
Today, there's suffering, pain, crisis, despair, and many other horrible things that people go through every. If we can soften that with "have a good day" why wouldn't we?