Why do some people ignore people they know when they see them in a store?

I sometimes ignore them because I forgot their name, and I don't want to be in a awkward situation. When that happens I say "Hi? How are you?" and such.

Depression, sometimes you can feel like other people can sense it more when you are some where out of your element.

I do it sometimes when I am engrossed in shopping or looking for something in the store. But that's not on purpose. Normally I acknowledge everyone I know with a smile and maybe a greeting. Unless I don't like them. That's when I deliberately ignore them.

I do that all the time - It's not because I want to be rude or unfriendly, it's 2 main reasons.

1) I have an eyesight condition, I can see just fine but peoples faces can be a bit of a problem until I get pretty close - or I can stair at them wonky eyed from a distance till I focus.... You tend to get pretty self conscious about that sort of thing after about a billion mis-identifications of people - so I prefer not to go around staring at every face just incase they think I might want to kidnap their cat or something.

2) Usually when I am walking around in a public place I am not there because I enjoy dilly dallying around - if I'm in a shop I know what I want to buy, if I am in the street I know where I am going - so this walking time is usually semi automated and I am thinking about other things.

I AM NOT RUDE!!! People who choose to think I am can think what they like, If they call out I will stop and say hi - I'm never to busy (usually) for pleasantries! - just some people are always looking out for human contact for some reason, some others either take it or leave it, I am the later