This is true to an extent. Happiness is very complicated on its own. But using measures of health, socializing, and overall satisfaction with their life it can be observed that many people are extremely happy alone and choose to be single because it makes them happier. According to Time Magazine, it matters the situation in which you as a person see the world. They state, "marriage doesn't necessarily make you happy...more likely, happier people tend to get married". So it depends on the person and what they believe will be fully satisfactory in their life. If someone wants a partnership but doesn't have one, it can lead to being "less satisfied in...life". Also, single people are reported to be more social, which many scientists say is important in the full happiness of life. So, yes single people can be happier but mostly it's circumstantial.