I think this is the essence of the question that the OP asked. Again, I think people understand the desire to sit next to one's travel companion; what I don't understand is the need to do so, to the extent of poaching seats or guilting people to switch.

I understand that you think certain seats are as good as others. But people have their own reasons for preferring one seat to another, and that reason isn't any of your business. You're an adult -- if you can't sit next to your travel companion for a few hours, you'll survive just fine.

A quick summary (as I see the issue) is as follows:

(1) Don't poach seats. Sit in your original seat and ask someone to switch, offering them the better (in their opinion) seat, if possible. People respond much better to your taking something of theirs if you ask first!

(2) If the answer is "no," accept graciously, and apologize for the inconvenience. You aren't entitled to know/understand why they refuse to switch seats and don't have the right to make them feel guilty for doing so.

(3) If you can't sit next to your traveling companion, understand that you will both get there at the same time, you are both adults, and you will survive the ordeal. You'll likely be spending every waking/sleeping hour with your companion at your destination, so a few hours apart won't kill you.

At least, that's the way that I see it.