There are various stages of mental health. People are often ill for a long time before others notice. This is because mental illness symptoms follow a progression.
People do not wake up one day and discover they are mentally ill. Part of the diseases involve attacking the parts of the brain that are responsible for self awareness.
Instead, people drift through life and have a series of failures. These failures might be work or education related, or they might have to do with relationships.
When a person continues to experience these disappointments, the person will probably react either with indifference or inability to change the behaviors.
This is why it is important for other people around the person to notice that the person is having symptoms. More awareness will make the stages of mental health and its decline easier to manage.
A person deserves a chance at fighting their biology. This is why other people need to be aware of mental health.