How to teens prevent a loss of interest in life?

Treatment. Loss of interest in life is seen in depression. Depression is a serious medical problem that requires treatment. Treatment should include both therapy and medication. I would strongly encourage you to ask your doctor for a referral to a child psychiatrist.
Teens. Involve them in meaningful activities that they would excel at & compete at, with continued positive reinforcement & encouragement as well as rewards for achievement. Group activities & competitive sports are vital.