The Mishna is understood but is very hard to fulfill. The verse in Mishlei says that showing someone the white of your teeth*** is better than giving them milk to drink. Greeting people with a big smile and happiness can make a huge impression. Our sages says that someone who gives a poor person money receives blessing but someone who appeases him with words receives even greater blessing (even if he doesn't give any money). I am sure everyone can think of some time when they were down and distraught and someone came by and gave them a warm greeting which help to cheer them up. We have such a tremendous opportunity to make people feel great when we see and meet them by greeting them cheerfully. This costs us nothing, but takes a great investment on our part to be aware of the people around us and their feelings. Although it might be hard to greet everyone with happiness we can all start by making sure at least once a day to greet someone with happiness. It could be your spouse when they come home from work, or your child when they return from school or a total stranger on the street. You never know what the other person is coping with in life and a sincere happy greeting can go a long way to brighten up their day.