How or why is Dante sympathetic to some sinners and not others?
I like Dr. Fajardo-Acosta's (of Creighton University) analysis, excerpted here (a link to the full lecture below):
"Dante's love and sympathy for several of the souls in hell acts as a form of redemption for them (e.g. Paolo and Francesca in the circle of the lustful, Canto 5; Brunetto Latino in the circle of the sodomites, Seventh Circle, Canto 15)Dante re-evaluating each sin and each sinner, in many cases forgiving what the Church could not; humanizing the sinner and showing understanding and tolerance of human passions; Dante imitating Christ and his forgiving and redemption of humanity: allusion to legends of Christ entering hell after his death and rescuing a number of souls (Adam and Eve, the prophets, etc.)
Dante's mission is re-redemption of the world by recalling and imitating the love and mercy of Christ."
As for Dante's lack of concern or anger towards certain sinners, Acosta addresses this as well: problems of Dante seen in his wrath in the circle of the wrathful (Fifth Circle) when he is enraged at the sight of Filippo Argenti, a personal enemy; Dante feels no sympathy for Filippo's tears and suffering and even desires to see him suffer more (Virgil too is caught up in cruelty and desire for vengeance)problems of Dante especially visible in the Ninth Circle of hell, the frozen, circular lake of ice at the bottom of hell and the residence of Satan."