As cynical as it may sound, it could also be an ego boost, an attempt to look more alpha, or self-righteous judgemental thoughts in the guise of tough love.
Some people are so socially brainwashed that if you're an anomaly to what 'their' idea (the most common one) of the perfect life, you're flawed or weird.
Either way, I can't help but pair such behavior alongside the likes of High School dabble/gossip. Some people just never mature or think one's belongings or success (in this case, relationship goals) is all that matters.
In the end, just do what makes YOU happy or keeps you sane. Even if that means being single. Forget what anyone thinks of you because in the end, they AREN'T you.
I was a third or fifth wheel for a large portion of my adult life. And I often got the same exact treatment. Sometimes at least weekly. To be fair, that is a drawback to hanging out with friends who don't have similar mindsets or interests. So yes, I admit there is some bitterness and bias in me. Doesn't help either that over time, as us (the trio) would hang very often and worked at the same workplace; feelings developed that constantly made two of us feel awful, conflicted and/or guilty for feeling such a way.
Anyways, to make a long story short there is some very real negatives for being a third wheel besides the ridicule or feeling like the odd one out. My best friend and his girlfriend (now, wife) and I began to have very real feelings and extremely deep and thought provoking conversations. I genuinely fell in love with her and I honestly felt she felt the same. Either of us could never admit it because it would tear it all apart. He had financial security and was more socially acceptable, I was the one she would lay her feelings, head, and thoughts on. The former usually wins. Not their fault, just how the world is made to be right now.
Fear and non-acceptance can make one's love narrow instead of open. Either way, I now purposely distant myself from them both. It's double the heartbreak for me to be around them these days. I'm sadly used to that by now anyways, reminiscing and loving from afar as I feel and hope it's for the best.
Strangely and thankfully, when I was dating my last girlfriend, those strong feelings of her were entirely suppressed. I guess some people can only love one person at a time on that level which is why it's usually "couples"?? Now that I'm single again, I constantly fall into and get lost into different former love pits in my mind. I'm sure some of the lonely here can relate to that. Feelings re-surfacing in an instant whenever you meet anyone with similar features to any of the loves of your life... Heh. Can't someone simply 'un-love' someone??
So again, I reiterate, there is some very real negatives for some when you're single long-term and still very social-able. Higher risk of falling in love with one of your close friend's SO's is a very big one. Congrats to those that don't fall under similar circumstances. I'd be lying if I said I wasn't envious.
Sorry for going into a ramble, just figured I'd be thorough with my views and experience. I also selfishly needed to get that latter bit off my chest..... (sorry OP)