Unless you yourself have gone through a mental breakdown of any kind or suffered through a mental illness or even know of someone who is suffering through it, you can't really say that you disregard them. With experience (first-hand or second-hand), comes knowledge and understanding.
There is already a stigma with mental illnesses and people not believing when someone says that they are going through something mentally-related, it makes it even more hard and exacerbates their condition. It's better to give the benefit of doubt then to discredit them.
Obviously you cannot be self-diagnosing yourself, that is very serious business. I know that if you go to a hospital and say that you sometimes have thoughts of hurting yourself or others but you can control it, even then, the doctors or residents on call will keep you overnight against your will. I know so because it happened to me and I was so angry, mad and depressed that I could not go home with my wife and stay with my parents. But I had to oblige to their request or I would have been restrained in a straight jacket by the orderlies, which would have made me go ballistic.