Smile, be happy, be kind, be polite. But don't expect the same in return, these things are not requirements, everyone is entitled to their own attitudes towards life, if they don't want to smile, so be it.
If their not smiling affects your ego then that is not their problem. We all have egos, active, sensitive, offendable egos. Be true to yourself, if you want to smile, to say hello, awesome.
If you don't, oh well.