Why do scientists volunteer to be involved in public engagement in science? What are the barriers that can prevent them participating in dialogue with society? What can be done to facilitate their participation? In this paper we present a case study of the Children's University programme of the International School for Advanced Studies (SISSA) (Trieste, Italy), discussing the three-year experience, and reporting the outcomes of a series of focus groups conducted with the young scientists who volunteered in the programme. Two kinds of motivations emerged. The first is personal, for example volunteers' desire to improve their own communication abilities, or their curiosity for a new activity. The second is related to the perceived role of scientists in society: many volunteers feel a sense of duty and the need to promote science and its importance in society, to have an impact on the public perception of science and to seed the love for science in young people. After the first year of their involvement, volunteers expressed the need to keep improving their communication skills and participating in professional training courses, and agreed that science communication should become part of all standard training programmes of PhDs. In order for the outreach not to remain a sporadic experience, it is essential that a strong institutional commitment exists to promote, recruit, encourage, professionally train and support those involved.