1. Everyone is affected by it
Everybody in the world has their own set of emotions. Knowing how these emotions affect someone on a personal level will help people understand one another and help create better unity. Having knowledge about mental health can help not only you, but someone who is dealing with their own personal problem.
2. Being mentally healthy will increase quality of life
Knowing what's going on when a situation occurs will give anyone an added peace of mind. It's no different for mental health. Knowing the signs for when something out of the norm is happening will lead to just enjoying life with less worrying.
3. People find mentally healthy people more enjoyable
People tend to enjoy being around people who are positive, don't over react in situations and are friendly to approach. These people tend to have good mental health. Learning about what good mental health is can lead to more being enjoyable to be around.