Psychology journals are overflowing with articles about loneliness, and have been for decades. Recently, scholars are starting to study solitude, in the sense of the positive aspects of being alone. Take a look at what we know so far. Also take a look at Alone: The badass psychology of people who like being alone.
The assumption that if you get married, you will get healthier is so much a part of our conventional wisdom that it is rarely challenged. Back when I was just practicing single life and not studying it, I had no idea that the supposed truism was actually a myth. I figured that out fast, though, once I started going to the original research reports and scrutinizing them. I drew from what I had learned from decades of doing research and teaching graduate courses in research methods, but some of the mental errors in the claims about the research are so egregious that you should not need any formal training to realize how ridiculous they are.