It's impossible to know what someone else is feeling if they haven't shared it with you. But I propose we try to treat every person (friend, foe, stranger) as though they were our best friend. Treat them with kindness and give them the benefit of the doubt. Give them a sincere smile and say thanks when you can. Go the extra mile.
I think if we all did this, all of us would feel less alone in our daily travels. Plus, you never know how truly lonely a person may be on the inside. And who knows, you may even brighten someone else's day, not just your own.