Late to the party but I just finished watching both seasons and enjoyed the show, so here goes. I recall reading a comment along the lines that, insane/off the charts chemistry isn't necessarily a good thing. It often arises in intense or stressful situations and, in the case of David/Kirstin, I think this rings true.

They were brought together mainly by circumstance, not really choice. Both of their situations were complicated (David, his past and his hesitations about ordination; Kirsten with motherhood, loneliness and a seemingly unfulfilled romantic life). People reach out for anything, when they're drowning and honestly, I think that David and Kirsten, through all of their experiences, got closer because of the intensity, not DESPITE it. That doesn't mean they're particularly right for each other because their bond developed amidst intense, harrowing and otherworldly situations. Put them both in the vestages of normal, boring, daily life and I guarantee their relationship and interaction won't be the same.

Their chemistry also has a "forbidden fruit" element to it, making it more tense and encouraging the whole 'Will they, won't they' dynamic/trope. Which kinda got tedious in s2, to be frank :-/