Being single is as much a state of mind as it is a state of relationship. It covers a whole range of situations, you can be young or old, you can be in 0, 1 or more casual relationships, you may have no children or may have children from a previous relationship, you may be searching for a new relationship, or be perfectly happy being single and intend to remain in that state.
Under any of these circumstances, the main thing is that you are able to appreciate the world around you, and enjoy your life as it is. Of course, this is not always easy. A solo parent (male or female) with young children tends to be exhausted from the effort of catering for a young family by themselves, and often stretched financially by their circumstances. However, if they are able to establish a social network around themselves such as a group of family or friends, then they have the opportunity to use this support network to provide enough child-free time to have some time to themselves for enjoyment, without abandoning their children or putting them at risk.
Everybody periodically needs some time alone, away from others, in order to reflect and consider their place in the world, where they are going and what they are doing. Married or single, you occasionally need to take this time to focus on yourself, and decide what you want to achieve in life and what is realistically attainable, and put some plans in place to start heading there, since nothing will just happen by magic. However, you also have to make a personal commitment to enjoy life as it currently is, find the parts of your current life that are special and valuable, and cherish those.
When you are single, you have the time and the opportunity to get out and meet people, to experiment with different lifestyles, to experience different relationships in a way which becomes more difficult (but far from impossible) if you are married or in a settled relationship. While all of these things take time and effort, and have a stress all of their own, all of them have their own rewards in the form of the people you meet, the new things you learn, and the new experiences you gain.
As you get older, being single is often more difficult, especially if you have previously been in a settled relationship that was interrupted by divorce, separation or a death. Now, most of your attitudes and habits are all based on the life that included the relationship, and hence tend to involve automatically considering the other person who used to be in the relationship. It takes a while to get used to the fact that it is no longer necessary to consider that second person, and to re-plan your life around being single again. However, once that mindset adjustment has occurred, the single life still offers many opportunities for enjoyment, fun and experience. Of course, the ending of the previous relationship will almost always also include a period of grief and mourning over the circumstances that caused the termination of the relationship, and grief passes in its own slow time.
However, once you have accepted that you are single, and once you start actively looking for the many things that can be enjoyed and appreciated in that single state, then the opportunities for new life, new experiences and new enjoyment are boundless!