@ DeadGirlsDance
Part of the answer is in the question, so-to-speak.
A quick web search turns up:
Chav (/ˈtʃæv/ CHAV) is a pejorative epithet used in Britain to describe a particular stereotype. The word was popularised in the first decade of the 21st century by the British mass media to refer to an anti-social youth subculture in the United Kingdom.
SCUMMY
1.getting seriously intoxicated, usually off a combination of marijuana and alcohol.
2.the way you look after a long night of getting wasted, usually dirty, nasty.
1.Yo, you wanna roll? Me and the boys are getting scummy tonight.
2.Them hoes look scummy as hell!
Now, the "social smile" is one of the earliest milestones in child development, causing concerns if not developed at the expected age, which long before a baby learns to talk, walk, sit up or feed himself or herself. But to some extent, even babies are choosy about whose smiles at them they reciprocate, and have to be taught to adopt this superficial friendliness indiscriminately, a skill that many lose later in life.
Because the babies haven't learnt to talk yet, we cannot ask them their criteria for deciding whom, amongst the majority of the population who cannot help themselves smile at babes of a certain age. However, you have given us (and yourself, if you read what you wrote) a strong clue what *your* criteria are for rejecting the social smiles of others. The people who smile at you, whose smiles you do not return, are more likely to be male than female. They are more likely to be old than young. And they are people whom you have judged, assigning them, in your mind, to stereotypes, such as grubby, tired, drunk and stoned ("scummy") or anti-social youth ("chavs").
So, the people whom you offend, by not reciprocating their social smiles, unsurprisingly turn out to be the sort of people who make comments that are intended to let you know that your behaviour falls short of their social norm expectations.
If you always smile back at everybody who smiles at you (like some babies do), then nobody will ever say anything like that to you ever again. If there are certain perceived demographics (or stereotypes) that you dislike enough not to smile back at the smiling strangers concerned (about all of whom you given away that you make instant judgments, such as estimating their age, gender, chaviness or scumminess), all of the minority of those offended by your loss of the "social smile" skill that marked a key stage of your child development before you learnt to walk, talk, sit up or feed yourself, about the time when your diet stopped consisting entirely of breast milk in fact.
Learn the "dismissive smile" that people give one another on the Tube in London, whenever accidental eye contact is made between random passengers. The smile that says, better than words, "oh, we've seen one another, and we are merely strangers, not enemies; so I suppose we'd better both give a token smile; it's the custom; but neither of us is up for a 'getting to know one another' first conversation; no need to exchange words" I must have given and received a dozen or more dismissive smiles like that, on the Central Line today. Back home, in Cornwall, people don't do the "dismissive smile" thing very often. Cultures vary. Most often, it happens when a man who doesn't feel old, and has forgotten that he looks old, smiles at a young woman.
Try it. A dismissive smile takes only about a fifth of a second, uses negligible calories, and prevents offence. If that doesn't solve your problem, I don't know what will, but at least your problem is not as bad as some of the other problems people have, especially in other countries, where people often have very little to smile about. As you have said, you are "happy".
By the way, I joined this forum because there was a topic I wanted to comment on several months ago. I'm not actually a student. I am a grandfather of eight. Hence the "lecture". :-)