I think Sondheim is jealous of Hart (whom I consider to be the greatest lyricist ever). Hart lived in a time when cleverness was the most-prized virtue of a lyric. Hammerstein's strong suit, on the other hand, was the emotional romantic song, such as The Folks Who Live On the Hill or If I Loved You. I've actually not heard Sondheim say much in praise of Hammerstein, the lyricist, although he obviously appreciated Hammerstein the person. Mentor. Teacher.
Here's my controversial thought of the day: Sondheim's strong suit is the clever, not the romantic. I'm not saying he's never done the romantic well, but my favorite Sondheim songs are the most Hart-like - You Can Drive a Person Crazy, You're Gonna Love Tomorrow/Love Will See Us Through - that sort of thing. Sometimes, when he attempts to write a love song, he seems to trip over his tongue. What's a word like "implacable" doing in an expression of ardor? "Crazy business this, this life we live in" just makes me choke. Nobody talks that way; it's not romantic to do so.
But I certainly "felt the love" in Sunday in the Park With George, which is about an artist who is unable to express his feelings, except through his work. So, nothing that George says moves me - he's intentionally inarticulate, emotionally distant - but when his daughter points out that, in the painting, "Mama is everywhere. He must have loved her so much," I cry buckets.
In a way, much of Sondheim's career has been a rejection of what Hammerstein taught by example. We see couples falling in love and getting together in South Pacific and Oklahoma. We see couples rushing to divorce in Merrily We Roll Along and Follies. It seems Sondheim is more interested in depicting unsuccessful liaisons, unlike Hammerstein.
And sometimes, as an audience member, I'm left unmoved by Sondheim shows. I think that there's some truth to his reputation for being cold: Pacific Overtures and Assassins were interesting history lessons, but not emotional experiences for me. Passion seemed ironically titled. Even in the most gorgeous of his shows, A Little Night Music, the characters seem to be more motivated by sex than love.
I admire Sondheim most as a songwriter, not as a show writer. That is, there are a great number of his songs that I think are wonderful (and, yes, emotional and romantic). But there are just a few of his shows that satisfied me, as a whole, in the theatre.