It talks about it earlier on, basically love is nebulous and can be temporary, Yamori-kun is on the right track but intense situations or pain create a suspension bridge effect and push him over into vampire form
Intense situations? Hasn't he needed to bleed each time to actually turn, and that's why he uses the ear piercer to turn? Else when Kiku was drowning him he should have been able to turn eventually, not to mention a fee other intense situations before he first turned.
Like, I do remember the point about the suspension bridge effect, but I understood that more as them trying to find an explanation to how the mechanism for him turning works, not so much what actually enables it to work in the first place.
Nah, nah, nah. Continuing my thought back from the day I still believe they worrying too much about situation where vampire bites the person they love. Let's go through what we know:
1. Mahiru definitely loved Kiku, therefore when Kiku bit him, he transformed into a vampire.
2. Mahiru died back then, thus we can conclude that sun is deadly for vampires.
3. Kiku was in the same spot as Mahiru, so the sun beams were falling on her as well.
4. Lastly, it wasn't the first attempt when Kiku tried to fall in love with someone to become human.
In the end, considering the last fact, we have two equally possible outcomes of their meeting:
1. Kiku turned into a human and died because of fast aging. Mahiru turned into a vampire and died because of the sun.
2. Kiku failed at becoming a human, but decided that it would be better to die here and stop her suffering at once instead of trying again. So they both died because of the sun.
Again both of these outcomes are equally possible, so there's literally no reason for characters to worry so much.
I thought they had stated that sunlight isn't effective for killing, just for weakening them - Nazuna's mom and Kiku discuss it, from what I remember, on the flashback with Haruka/Hatsuka/the guy that Nazuna now treats as a brother.
The most likely scenario would be that drinking the blood of someone you love might be fatal, or how Mahiru drank Kiku's blood so soon after turning into a vampire and then exposing himself to blood...
or even more tragically, the bokuto that Kou gave Mahiru being so close, while having Kiku as human in front of him (basically bombarding him with memories of being human between all that), made him weak enough that as a newborn vampire sunlight was fatal.
Something similar was how the detective killed the vampire guy on the school way back, after all (and he was weakened instead from a lack of feeding during years).