In general, Swedes do not say hi or start up a conversation with someone they do not know. Let me explain this with a real life example.
I am from a very small town in Sweden, and one day on my way home from university, all of our trains were canceled, so I had to call my dad and ask him to come pick me up. I recognized one of the women on the platform, because she works in our local grocery store. So I walked up to her, as the non-conforming Swede that I am, and asked her if she wanted a lift home.
She looked at me in utter disbelief. Why? Why would you offer me a ride home? She seemed genuinely shocked. I explained that I recognized her, and I thought I would offer her a way to get home since it was freezing cold and the replacement bus would not arrive for a few hours.
Somehow she accepted, and in the car on our way home, it turned out that my dad knew her from teaching soccer in our small village. Yet, this was still a very strange experience for everyone involved.
Having lived abroad for such a long time, I found it quite surprising that I could not just go up to a stranger (well, she was from my small town), and talk to her about our shared problem with the trains. Whereas she found it shocking, which she later told me, that I just offered her a ride - for no reason.
This situation perfectly explains the Swedish culture of non-interaction. Most Swedes are very reserved, and solving problems independently means less chance for awkward interactions or strange social situations.
So if you are wondering why Swedes are not returning your friendly hej’s and nods, it is because it is simply not in our culture to be friendly with people we do not know.
