Being sad, stressed or depressed or having other difficult feelings is a normal part of life and having these feelings does not mean someone has a mental health problem. All sorts of things happen in life and these feelings are a natural reaction.
However, if someone is feeling constantly unable to cope, having feelings or experiences that are “taking over”, or having feelings that can't be understood or can't be shifted then this could be a sign of a mental health problem. One in 4 of us will experience a mental health problem like depression, stress or anxiety at some point in our lives and there is good treatment and support out there to help people get better.
Take a look at the Young Minds website for more information.