The above linked articles talk about anxiety. It is difficult living with anxiety and difficult for those living with the person to cope with it, too. Telling the person “just relax” is not helping the situation at all. It’s really not a choice, but just the way the person’s brain works.
Some days are good, some are bad. It depends on many things. Stress level, amount of sleep, diet, medication, etc. It varies from day to day. Each person copes differently. Different things work for different people and the level of anxiety is different from person to person. Many people try to hide their anxiety, but those who know them well can usually tell. Be patient and use tools that help your loved one, after you find out what works for him or her.