Thanks for this, and to @BiologicalKitty. In my donkey's years of analysing relationships, in therapy and elsewhere, I'd never clocked this as a significant issue. I damn well should have, looking back at the number of times I've yelled "It's not a competition, you know, we're supposed to have each other's backs!"
Competitiveness is a factor in all human activity, I think, but I'd overlooked the depth of its influence - and men are more competitive than women, it's one of only two behavioural characteristics that can genuinely be ascribed to testosterone (the other is impulsiveness).
Couples often feel well-bonded when they're 'competing' together. Whether it's dinner parties, herb gardens, tastes in music, money or sport, we all navigate tides and eddies of social positioning. Joe & Jane might come away from Bill & Bea's party full of admiration for their barbecue setup, yet feel great about the fact that their children play musical instruments while B&B's kids only play the fool.
If J&J can't find enough ways to think themselves 'better' or equal to B&B, Joe's likely to take it harder than Jane ... and quite possibly blame her. Other reactions are available, including a recognition that garden parties aren't a zero-sum game, but I suspect that one requires an inner self-confidence that's in short supply.
In my dysfunctional relationships, I have acknowledged that I was often looking to get one up on my partner during rows and stand-offs. I've viewed this as a fault and couldn't quite understand it, as I'm only averagely competitive and certainly didn't wish to escalate the arguments. Were I to go back over them, I bet I'd find he was trying to boost himself by metaphorically destroying me, and that would be game over. My mistake was playing this 'game' instead of walking off the field the second time it happened.
I guess some of the conclusions are that Mr Evolved Masculinity only competes where it's appropriate, likes being on a team and sees his wife as co-pilot, and is relaxed enough to let everyday rivalries wash past him.
Having mixed a ridiculous number of metaphors, I'd like to thank MN for putting up with my ramble and for this useful bit of insight!