Some people, including women, like to feel connected to the people around them, even if only for a fleeting moment. If you live in a very rural area, most people will smile and nod at you, although that's dying out there too. In cities or towns there are too many people to give eye contact to so you can only pick a few at random.
It gives people a slight emotional boost inside to acknowledge others and be acknowledged. Conversely, staring at nothing while moving through a faceless mass of humans makes many feel lonely and sad.
People also do this to defuse their surroundings. Maybe you look sad, so they smile in the hopes you'll reciprocate and feel slightly better. That may not even be very conscious by the way. However, I would hazard a guess that this is bog standard eye contact.