Mental illness is a terrible thing. But with the help of professional treatment, people with this illness can overcome it and live happy, productive lives.
However, if you have a friend or a family member who you care about who has mental illness, you can't always help the person on your own. Often, mental health providers are needed.
For example, if you know someone who has bipolar disorder, and if the individual is in a manic phase who has lots of energy with grandiose ideas that aren't rational and realistic, trying to reason with him /her can be a bad idea. The reason is because if the person is manic and you try to reason with him or her, he/she might get quite angry at you. Those in a manic phase often can't be reasoned with.
If the person could be a danger to himself or herself, the person needs to be put in the hospital in order to stay safe. But attempting to reason with a manic individual while he/she is sick by trying to bring them down to Earth is unlikely to work and could make matters worse. Instead, it's better to let the medication do it's job first. Then once the mania is treated medically, the person will become much more rational.