The best reason of all is that you're happy like that. Some people do feel happiest living alone - they may even have relationships but prefer to remain independent. A lot of people now like to combine both worlds with, for example, LAT (living apart together) partnerships. Or you may like to be completely single. If you feel happy and fulfilled being alone, you should think very carefully before you change that.
The second reason is that you haven't found a person you can be happy with. It's so easy to feel pressured into settling down with someone who seems 'OK', but you will probably be less unhappy on your own than with someone who isn't right for you. So again, think very carefully.
A third reason is that, at least for a while, being on your own is very good for you. Everyone should probably live alone for at least a year at some point - you learn to be independent, to make your own decisions and to entertain yourself. All this will make you a much better partner if and when you eventually decide to get into a relationship.