From his Breviloquium (5.1.5):
Now no one possesses God without being possessed by God in a special way. And no one possesses and is possessed by God without loving God and being loved by God in a particular and incomparable manner, as in the case of a bride and groom where each loves and is loved by the other. And no one is loved in this way without being adopted as a child entitled to an eternal inheritance. Therefore , the 'grace' which makes pleasing' makes the soul the temple of God, the bride of Christ, and the daughter of the eternal Father.And since this cannot occur except through a supremely gracious condescension of the part of God, it could not be caused by some naturally implanted habit, but only by a free gift divinely infused. This is most evident if we consider what it truly means to be God's temple and God's child, and to be joined to God as in wedlock by the bond of love and grace.
[St. Bonaventure, Breviloquium, Monti, ed. and tr., Franciscan Institute Publications (Saint Bonaventure, NY: 2005), p. 172.]
There are probably a number of reasons why St. Bonaventure is often called the Seraphic Doctor. One is his emphasis on divine love. Another is probably that the Franciscans wanted an epithet for him that would sound more impressive than that of the Dominican St. Thomas Aquinas, the Angelic Doctor. (One can never overestimate the power of the Franciscan-Dominican rivalry.) But a major reason would be his theological explanation of St. Francis's baffling vision of the Crucified Seraph. From the Itinerarium:
Inspired by the example of our blessed father, Francis, I sought after this peace with yearning soul -- sinner that I am and all unworthy, yet seventh successor as Minister to all the brethren in the place of the blessed father after his death. It happened that, thirty-three years after the death of the Saint, about the time of his passing, moved by a divine impulse, I withdrew to Mount Alverno as to a place of quiet, there to satisfy the yearning of my soul for peace. While I abode there, ponderin gon certain spiritual ascents to God, there occurred to me, among other things, that miracle which in this very place had happened to the blessed Francis -- the vision he received of the winged seraph in the form of the Crucified. As I reflected on this marvel, it immediately seemed to me that this vision suggested the uplifting of Saint Francis in contemplation and that it pointed out the way by which that state of contemplation can be reached.[St. Bonaventure, Itinerarium Mentis in Deum, Boehner, tr., The Franciscan Institute (Saint Bonaventure, NY, 1956), p. 31.]
The rest of the Itinerarium goes into the details. I've previously given a summary of the first two of the six wings of the Seraph.