When the two songwriters stopped writing together, one went off to do songs that tended to err in the direction of being too sappy, too happy, and the other erred toward too angry, too weird.

When they worked together, they tended to not err. They also didn't repeat.

Two people with different imaginations when working together pulled each other and held each other back just enough. They got ahead of people in general, but not too far ahead. They got a little ahead, made music there, and whoosh! lots of people were in the new place and liked it.