Apart from 'by choice', some of the reasons some of us are single (still) is that we are always with the wrong person either by choosing or misfortune. Also some of us are always 'deserving better'. Whenever the heavens gives us a good offer we are always saying "we deserve better" (Some of us have been on it for decades now). Another 'can be hard to believe' reason is that some other people don't want to see you happy.
May the heavens have mercy.