I think it's a little of everything, if you ask me.
Obviously the low birth rate and population drop means there are just straight up less people, especially in the countryside, but also obviously there's still enough people to make Tokyo among the most population-dense metropolitan centers in the world.
Japanese society itself strongly eschews individualism in favor of uniformity and conformity, and there are many social etiquettes and mannerisms that further complicate human interactions and relations. Depending on how social and expressive you are, it really can be tiring enough that you might want to avoid extraneous interactions.
Japan has also been fond of mechanical constructs since time immemorial. In ye olde days it was karakuri ningyous (mechanized puppets), and in more recent times it has been robots ranging from the cute (Doraemon) to the super (GaoGaiGar, et al.) to the fantastical (Evangelion, et al.) to the "real" (Gundam, et al.) to really advanced vending machines and companion robots (eg: Aibo). Japan simply doesn't share the west's fear of machines (read: Terminator, HAL, et al.).
In short, while the west holds human interactions and relations as absolute necessities for a proper society, in Japan they entail a lot of social baggage. A lot of Japanese happily prefer the warm glow of a vending machine to a cashier, as sad as that might sound to many here.