How does taking command put extra strain on marriage?
It's the extra responsibility - the buck really does stop with you. Additionally you have to deal with a new set of issues. It settles down after after a year or so depending on your company culture.
The best advice I had was 'don't look at the right hand seat and expect to see yourself'. I.e. your newly acquired colleagues in the right hand seat won't necessarily behave as you expect! When I swapped seats ninety percent of the First Officers were bloody brilliant. A few were very hard work . . .