As other people have been saying on here, Andy feels less pressure playing doubles and is more relaxed.
I bet if he and Jamie became a permanent pairing that pressure Andy puts on himself would probably come back. He'd still be wanting to win tournaments, get to the top, etc - ambitions his brother might not share, especially as Jamie has other priorities in his life. Anyway I think Andy's too restless a spirit for that to happen, just now at least.
Anyway, just think - what would the dear old British media do about their "75 years" (and counting) obsession? Transfer the country's expectations to our No.2, Bogdanovic?