Some people are just naturally grumpy. Everyone who is in close contact with grumpy people is affected by them. One grumpy person can have a huge influence on family members and relationships. After years of living with a grumpy person you can usually take them in stride as seen in the movie "On Golden Pond". It takes a lot of time and practice. Another person's grumpiness can be very frustrating and interfere with your own mental health. I think those of us who are prone to depression and anxiety have more difficulty taking things in stride and we might easily over react to someone's grumpy disposition.
I'm a senior citizen and often hear younger people refer to an older relative as grumpy. Some people associate grumpiness with getting older. Are there a lot of grumpy senior citizens? I suppose health problems and getting older in general could cause some senior citizens to become grumpy. However, there are many people who are grumpy most of their lives. I have a feeling you could explain to a habitually grumpy person how their grumpy attitude affects others but it would probably cause a very defensive response.
It's hard emotional work dealing with grumpy people on a day to day basis. Before you start, make sure you have your own depression and anxiety under control. Feeling good yourself is a requirement for success when co-existing with the grumpy people. Be sure not to allow their grumpiness to overcome your positive attitude.