Guy asked me why I'm single made me sad

Exactly! There's nothing wrong with being single and there are many types of love besides romantic love one can enjoy and engage in. Society places a lot of emphasis only on romantic love, as if it's the end-all, be-all of all loves. We internalize this from birth. There are certain things ppl assume they are just supposed to do when they are adults like have kids, buy a house, and get married. "Milestones."
Black roses, if you want romantic love, I hope you find it. I also hope you know that you can live a happy, sexy, fulfilling life as a single person. Many people in this world are single.

This was a coca cola commercial but it gives me happy goosebumps every single (pun intended ) time.

I'll make an analogy. I had a co-worker who was married a few years. It was a good marriage, and the couple desperately wanted a child. They had been infertile, but were seeing fertility specialists to try and remedy the problem. My co-worker confided this to me, but she chose not to tell everyone at the job.

A superviser kept saying to her: "When you gonna have some kids? Don't you want kids? What are you waiting for?" Finally I told the supervisor to knock it off. I reminded him that there can be lots of personal reasons and that he should think about that. Well, a light bulb went on in his head, and he snapped. He was smart and basically decent. He said, "Oh, I guess you're right." He sounded a little embarrassed, and he never again queried the gal about her childlessness. This man was smart, educated and socially sophisticated. Yet, he was unknowingly tormenting that gal with his rude questions. It just completely failed to occur to him that this young woman's childlessness was not a choice, but was actually a heartbreak to her.

It is very wise to learn that some questions are wrong to ask. Some things are private and nobody's business. Some things are painful. I don't have a right to go up to someone, rip a bandage off a wound and think "I just gotta see what's under there!" No I don't gotta.

Hahaha it’s great! Hilarious. Especially the end when she is asking about kind of man Heather wants and heather is already gone to enjoy her life. Awesome

It might be also that he’s strange that she’s still single because of her virtues.
The fact that we take personal some comments is because of our own insecurity or because we see what we are told as a fault in ourselves.
Adding, in the OP case, the social pressure there’s still, especially, in some places, onto single people.

Starting from the idea that it’s a stupid question unless this guy was interested at having something with you, here, you have a chance to heal something that hurts and has an effect on you. You have to find a solution because you deserve to be happy.
Somehow, you have to overcome the negative feelings that produces the fact that you don’t have now a romantic partner.
It may seems nonsensical to you what I’m gonna tell you but as long as you more disengage the need for a partner, the higher possibilities you will have to built a healthy relation with a SO.

Why? Because you are gonna feel happier so you are gonna project this to the outside. You are gonna be able to choose well. You will see the other as someone who is gonna add. Someone whom share a nice adventure but you will always be yourself and be content with yourself.
Meanwhile, enjoy other kinds of love, from friends, neighbours, relatives, pets, new people to meet…

How to get it?
Doing activities you enjoy and enrich you, do little steps to attend to places where you can meet people (without any expectation a prior) only for the sake to share a little with them, fix your short-term goals to achieve and enjoy the time of satisfaction you get from getting these goals when you arrive home.
Practise self-care: Cosmetic, physical exercise, a look change,
Discover new things, new places.
If you have the opportunity and time to do something for your community, do it.
Have faith in you! This is the most important.