From Pope Benedict XVI, General Audience, 3 March 2010:
[St. Bonaventure] was called Giovanni di Fidanza. An episode that occurred when he was still a boy deeply marked his life, as he himself recounts. He fell seriously ill and even his father, who was a doctor, gave up all hope of saving him from death. So his mother had recourse to the intercession of St Francis of Assisi, who had recently been canonized. And Giovanni recovered.
The figure of the Poverello of Assisi became even more familiar to him several years later when he was in Paris, where he had gone to pursue his studies. He had obtained a Master of Arts Diploma, which we could compare with that of a prestigious secondary school in our time. At that point, like so many young men in the past and also today, Giovanni asked himself a crucial question: "What should I do with my life?". Fascinated by the witness of fervour and evangelical radicalism of the Friars Minor who had arrived in Paris in 1219, Giovanni knocked at the door of the Franciscan convent in that city and asked to be admitted to the great family of St Francis' disciples. Many years later he explained the reasons for his decision: he recognized Christ's action in St Francis and in the movement he had founded. Thus he wrote in a letter addressed to another friar: "I confess before God that the reason which made me love the life of blessed Francis most is that it resembled the birth and early development of the Church. The Church began with simple fishermen, and was subsequently enriched by very distinguished and wise teachers; the religion of Blessed Francis was not established by the prudence of men but by Christ"[1]
So it was that in about the year 1243 Giovanni was clothed in the Franciscan habit and took the name "Bonaventure".
[1] (Epistula de tribus quaestionibus ad magistrum innominatum, in Opere di San Bonaventura. Introduzione generale, Rome 1990, p. 29).