I think I’ve figured out why only the first season of Durarara really works. It’s a matter of philosophical & thematic consistency.
Durarara is actually a meditation on how morality is affected by intent, agency, and information. In the first season, literally every character arc and plot point comes back to exploring this problem. We are presented with an array of monsters — people with absurd configurations of intent-agency-information-action.
Our series antagonist’s motivation is literally one of the standard christian solutions to the problem of evil: he loves humans (and the entire range of human behavior made possible by free will) so much that he arranges elaborate tests that force them to grow and surprise him with their flexibility. This character is the only one in the series who really knows what’s going on most of the time, and he is a monumental asshole, but he is also charismatic & lovable because he’s driven by love.
The series’ three main protagonists are each the head of a group. Each group has a different level of internal control that can be employed, and a different level of informational feedback: a voluntary collective, a traditional hierarchical gang, and a remote control hive mind. At the beginning of the series, each of them has abandoned the group they lead, and in their absence the group has taken on a character they dislike & performed actions they disagree with — so they end up taking control and responsibility again, which pins them against each other. We see the kind of subversion possible in each, because this flavors our sense of the moral culpability of their respective leaders.
The show also explores how history relates to romantic love, by showing us two monstrous couples: one composed of narcissistic monomaniac stalkers who have essentially been each tricked into believing the other is the target of their desire, and another more honest and genuine relationship that nevertheless is embedded in a context of lies and torture. We see a situation in which the explicitly ‘fake’ relationship is happier and more fulfilling in some ways — a situation made possible by the warped monomania of the participants — but we also see real emotional connection, bravery, and strength grow in a context made possible by cowardly evasion. (Into this is introduced an amoral character driven by curiosity, but he is too easy & too cliche — he doesn’t get a big role.)
In the realm of exploring agency, we have two superhumanly strong characters, one of whom is driven by logic and one of whom is driven by explosive rage, but both of whom are ‘good at heart’. The failure mode of being driven by logic is shown: expectations can be managed in such a way that important actions are not seen as necessary until it is too late to take them; at the same time, we are not shielded from the negative side effects of emotion-driven action either.
Ultimately, the result of the experiment is that we find people with ‘good hearts’ — people who mean well and are driven by intentions we classify as good (even when they are very warped) — lovable, and we love them no matter what they do. This is the least interesting conclusion, and the most problematic, because of what it does to the series later.
Beginning with the second season, the formula remains the same but the focus is dropped. Our already massive cast of characters expands to include more quirky folks who perform morally complex actions. These characters are no longer illustrations of extreme moral complexity created by pushing the sliders all the way to one side, and so they are less interesting. Their arcs are no longer meant to explore this problem, so they feel aimless.
Where season 1 was concerned with how to invent a plot justification for analyzing the failure modes of voluntarism, season 2 was more concerned with how to invent a plot justification for putting Anri in a witch costume. While profitable, this eats away at the primary appeal of the franchise: once viewers get over the novelty of seeing a little bit more of characters they love, they will notice that they are beginning to love them less, because it was the context that made them interesting.