The biggest disease of the mind is over-thinking, especially too much thinking about others: What they did, what they should do, what they should have done, what they said, what you wished they had said, why they spoke at all. All of these rob the mind of its inherent serenity.
Thinking too much is like eating too much. The heaviness makes it impossible to remain light and flexible. We get stuck on little things and gradually the little things become huge things which we cannot shake off. Often when we think too much we fantasise and overreact. Thus we create negative feelings.