In this talk, I will discuss why social relationships matter for us throughout life, highlighting how we need others for survival and how our lives are enhanced through our close bonds with others. I will show that even young children have a clear understanding of loneliness, offering appropriate definitions linked to loss, temporary absence, and psychological disconnection; they also have clear ways of coping with loneliness. I will discuss the importance of different aspects of loneliness, raising issues of measurement of loneliness, which are now the most challenging questions we need to address. I will also discuss interventions designed for lonely youth, and how the voices of young people need to be integrated during co-designed intervention development.