As a single person much of the time your married friends don't call to inlude you in activities. They feel awkward . That you might feel like the odd man out. Which is funny because most single people are comfortiable with themselves and don't feel the awkwardness. People's perceptions of things most of the time are wrong. Why because most people are not comfortiable with being alone with themselves. It takes a special kind of person to be by oneself. They are at ease with themselves and the world. Most people have to be busy or have people around them all the time. They can't be alone. They feel inadequate. Not whole. If people can learn to like themselves for what they are and not what they want to be, this World would get along with itself a whole lot better. We wouldn't have jealousy, pettiness, and greed on the scale that it is now.
I learned a long time ago how to be at peace with myself and not have to require the company of others. It's simple. Like who you are and not what you feel others think you should be. Stand up for yourself and like what you see in the mirror. If you can do that you will never feel lonly again. It's all in the mind and once you can learn to control that you control everything.