This claim can be more of an opinion than a fact. Everyone is different and thinks differently from one another, such as how someone prefers their companionship, relationship expectations, and connections. Someone could find the perfect person who matches all those reasons and more, and another person could spend forever trying to find just one good quality in a person and never find it, which makes it likely they will be happier alone. As stated in the article above, "Some people believe a romantic relationship is essential for their happiness and well-being, while others find fulfillment and satisfaction without a partner." This can, again rather, be proven as an opinion than a fact. Everyone is different, and there is no real factual evidence showing- "people who are single are happier."