The clerk at the grocery store is more likely to help you if you approach them with a "hello, can you please help me?"
The bank teller will complete your transaction faster if you show them a pleasant attitude.
This shouldn't be a shock to you, but the world does not revolve around you. If you need a moment to reflect on this, just think, will the world continue to turn and function once you have passed away? Most likely the answer is...YES! So with that being said, make your life and the life of others as pleasant as possible. The bank teller or store clerk may not know your name or what you do, but you might just make their day by being the only person who smiled and struck up a conversation with them. Think of making other people's lives beautiful and in turn your life will be just as wonderful!