We've swapped many times and have never met with resistance. We would only swap like-for-like, of course. But we've never even seen a pained look. All folks we've asked have been more than happy to switch.
Cheers.
Last flight I was on (762 jfk-sfo business) pax tried to change to be able to sit next to gf. He had a middle aisle she had a bulkhead aisle. Neither pax would agree to either change and they ended up flying apart.
In the scheme of things turns out it was an FTer whom I knew.
Last time I was involved in a seat change was when an old man wanted to sit next to his adult daughter and just took the seat.
My wife and I after being married so many years have no problems being separated. In fact we even travel to different places alone as one or the other of us may have a desire to go there and the other does not.