It is common knowledge in the western world, that sitting next to someone on public transport when there are other seats available is incredibly rude. I find it very hard to believe that someone who grew up in North America, Australia or the UK as examples would not know this. Exceptions would be, someone who is intellectually disabled, or from a far different culture.

Public transport is the adult world. It is not the yellow school bus or a singles mixer, and adult manners apply.

To assume someone would be crestfallen if you did not sit next to them, in a public space, when there are other seats available is shockingly conceited. It's not friendly, it's controlling, and it's putting your desires and needs above the other passenger's.