I see, but you 'no' see

We see strangers everyday. Some strangers are not absolute strangers because we tend to see them everyday e.g. in the bus/train while going to work, in the street, random person at work etc. There is no conversation apart from the odd 'Hello' when the eyes accidentally meet. You acknowledge that the other person exists. No one says a word. Not because they are bad or horrible, they may actually be genuinely nice people, but they are strangers.

When you happen to meet the same person again in a different place, what do you do?

Some people, like me, smile and say 'Hello'. They know the other person. According to them, their relationship with the person has changed from a stranger to 'this guy/girl from...' i.e. an acquaintance.

Other people just plainly ignore. They struggle, but still succeed, in avoiding eye-contact. Some people are adept in avoiding eye-contact. From their body language and facial expressions, you know that they 'know' but they still pretend that they don't 'know'. They recognise you and immediately pretend that they haven't.

Why? I do not understand this.

Is it:

a. fear
b. reluctance to talk until introduced
c. they just don't give a sh*t

Or am I missing a point altogether?