If you have high standards for yourself that you don't meet, you'll see what you want for yourself, calculate what it deserves, aspire to it, fail, then conclude "I do not deserve it, because I cannot be what it requires" and then reject yourself as a judgement against yourself.
That because you have failed to reach the standard you believe would make you worthy of love, that you do not deserve love.
And it is this failure to cross the gap between "what I am" and "what I think they deserve" which crushes you.
So brutally, the only reason some people are alone, is because they just don't think they're good enough not to be.
They don't believe they've earned the right to have love, and there is a type of agonising beauty to this, but let us not glorify it - it is one of the most tragic conclusions a human can come to about themselves.