Reasons that make people mean may not be generalized. I think being mean or not depends on personal attributes of an individual. People may mean as a reaction to frustrations in life. Some people may be mean because perhaps, they think that being soft and kind towards others may be misinterpreted as weakness.
becuase there depressed, people are still nice, you just have to search now, video games can make people moody, and people are also lazy, and rely on energy drinks to get up the morning and then they crash.