What's nagging at the back of everyone's head is existential loneliness. Self awareness, the ability to see yourself as a separate entity from the world, comes at a price: understanding that you experience everything utterly alone.
That is, you alone experience the feelings that emanate from within you. Sure, you can tell someone you feel sad, you can express joy. But you can't make someone else an integral part of what goes inside you. (Barring telepathy of course)
The togetherness we all crave is seeing how other people acknowledge, accept, recognize, validate our existence, and our identity and, most importantly, how we feel and experience life. That happens through actions, language, culture,... in countless ways really. And it gives us a sense of reassurance, acquiescence, confidence, trust,...
Rejection, then, is the awareness that the other person doesn't share that sense of reconciling their experience with yours. Either they have a profoundly different experience of life that doesn't align with yours; or they choose to live a different experience from yours.
Rejection is par for the course. Humanity isn't like the Borg. 7.8 billion lives each live their own individual of being alive, within their own bubble. You can recognize that you both feel similarly about each other, each within your own experience, and act on those feelings, or you don't.
The awareness of this disjointed experiences between you and the other causes discomfort within yourself. It raises questions. And above all, it makes it utterly clear that you are very much alone with yourself and your feelings.
What makes all the difference then is in how you deal with those feelings.
Regardless of all the advice, therapy, medication,... the hard truth is that you and you alone are confronted with the feelings and emotions that go on inside yourself. It's entirely up to you to face those and to figure out how to approach yourself.
You could do that through self soothing, positive affirmation of yourself, showing compassion and kindness towards yourself and so on. Or you could choose self loathing, beating yourself up, talking yourself down, blaming yourself, the rest of the world, the universe and so on. It's entirely your choice.
This has absolutely nothing to do with being rejected by others, and everything with how you decide to view and treat yourself.
Emotional maturity is about understanding this difference.
Having said that, it's important to recognize that people come in all sizes. Emotional maturity isn't 100% a choice. It's also determined by your mental health and your circumstances (upbringing, social economic situation, education, past experiences,...)
Even so (and this important) that doesn't mean you are excused from your personal responsibility as to how you deal with your own feelings and emotions.
Here's how that works:
> I think that's why incels attract such hate.
I mentioned that everyone looks towards each other for validation, recognition, acknowledgement and so on, right? This works both ways. If you choose to indulge in utter self loathing, and you decide to express that by behaving in a destructive fashion towards yourself and other people, well, the vast majority of people won't recognize their own experience in that... and simply walk away in an effort their own mental and physical integrity.
It's not "hate", it's called "guarding your own boundaries". And has absolutely nothing to do with discrimination.
You aren't being discriminated because you are somehow "less attractive". People simply walk away because they perceive you as emotional immature and your behavior as threatening.
Discrimination, however, is a different ballgame. That would be actively invalidating the experience and the existence of others through violent behavior. Simply because you feel, you are convinced, that the other doesn't fit in your own experience of life, your own view of the world, your own beliefs and you decide to act out on those feelings... And that's absolutely not what happens when you get rejected over a job interview, or a date.