The chemistry is that they are both unique at their respective art. Listen to the compositions of 'This charming man', 'How soon is now', 'The headmaster ritual', 'Stop me if you think...', 'William it was really nothing', 'Heaven knows I'm miserable now', 'Still ill' and 'Bigmouth strikes again'. From a compositional perspective and in terms of guitar playing level, NOTHING in the Morrissey solo canon matches this. The compositions and guitar parts of the solo canon are all nice and competent, but nothing you haven't heard before elsewhere. That's young Johnny Marr for you. This is one part that made The Smiths one of the best bands ever.
Then the lyrics and voice of Morrissey. His voice is like marmite but it's unique. Took me years to get used to, but I am addicted to it now. Morrissey's lyrics, when on form, are among the best, and they are unique and incredibly witty. Think The Queen is dead, Vicar in a tutu, her walkman started to melt, that's nothing you should hear me play pianer, Anthony said to Cleopatra, as he opened a crate of ale, sending frightening verse to a buck toothed girl in Luxembourg,
And the pain was enough to make A shy, bald, Buddhist reflect and plan a mass murder... the lyrics of How soon is now are exactly my life when I was a student. Inimitable, this man. Johnny Marr has played with many great bands afterwards, Talking heads are one of my favorite bands.... but it's not The Smiths.
Life is short and they should get the most out of it. But I respect they are not on speaking terms anymore and have no appetite to do the thing so many people want them to do. It's their life and they don't have a responsibility to make us happy. Still, I will never lose hope for a reunion, as long as they're both alive (well, in ten years time they'd be a bit too old for an energetic reunion).