I've only had this once, Husband was in 1K, I was aisle next to him, I forget where we were flying from but this works for us in First as we can share the view and still talk/pass things. A lady was happily set up with her Pjs and champers in my seat. FA asked her to move back to her seat on the other aisle. No problem, however she then went to great lengths to try and convince my husband to swap seats so we could sit together in the aisle and she could have 1K. Insisted we'd prefer it as a couple over and over. Lady, we do this a fair amount and know our preferred set up on each plane and flight duration. We both take windows for shorter long haul like Moscow and Toronto, then split on longer flights across the aisle. If we have our children (no faces please, they are adults) the children take our 1A and 1K and we take the middle. I'm really not sure why she thought she knew better but I do look a fair bit younger than my age and she was older so I assume she thought I was new to first.
Theres quite a few posts saying adults can sit separate on flights and wanting to sit together is infantile (implications to that affect). I cannot sit away from my husband or fly without him, I am extremely claustrophobic and over the years have developed my coping techniques, long haul I'm ok but flying Europe in business I find quite difficult and any unexpected delays can hit me (Recently sat at the gate for several hours with doors closed) A stranger wouldn't be able to respond to the increase in anxiety but my husband or either of my children are aware and can help me manage. Just food for thought.