Unrequited love really, really sucks. We can't help how we feel, it's just a part of our biology. If you're genuinely in love with someone, you obviously want what's best for them, but you're also distinctly aware that your feelings for them aren't what's best for them. And yet you can't help but feel that way about them.
If you're lucky, you know you're interested in someone right away. If that's the case, the best thing to do is just to confess; if you're rejected, it's not that big of a deal, you don't really lose anything. But it's not always that simple. A lot of times you fall in love with someone over the course of (or because of) your interactions with them. By then you might have a friendship with them that you value a lot and don't want to sour it with a potential rejection. If you don't confess, though, you'll keep wondering what could have been and you'll usually end up regretting it. Both choices lead to a bad outcome. If you do confess and you do get rejected, you might lose the friendship. But even if you manage to keep the friendship in either of these cases, then you have to live with the fact that the person you like is going to continue to pursue other romantic interests. And that hurts. And I mean bad.
So what to do? Most people are going to have to deal with rejection at some point in their lives, and in a lot of cases they'll have to deal with it before they're emotionally and mentally mature enough to handle it well. We should really try and prepare kids to deal with rejection early on, but they're never going to know just how bad it is until they experience it. What that means is that we need to be willing to help people who are currently dealing with rejection so they don't go off the deep end.
I saw entitlement being mentioned earlier. It's certainly true that nobody is ever owed a relationship, and I think you need to be firm with someone dealing with rejection so that they understand this. But you also have to understand that someone dealing with rejection is in a really bad place; they're feeling alone, hurt, and unwanted. In rare cases, it may be the worst they've ever hurt in their life. Being judgmental or scornful is not going to help them, it's not what they need. What they need is to know they're not alone, that other people have experienced this, and that they can get through it. That as bad as they feel right now, it won't be that way forever.
I'm going to go off track here, but this ties in and I think it's important. There's something really nasty that can happen right around this time to people who lack the kind of social support structure to deal with this. When these people are at their lowest and when they have no one they can talk to about these kinds of things, no one who will empathize with them, that's when they're at their most vulnerable. And sometimes, that's when they'll encounter a group of people out there who do understand their pain, people who are in (or have recently been in) the same exact situation. People who will empathize with them, will tell them it's not their fault and that they're still a good person. And these people will offer a very attractive solution: that the person you liked and people like them are the bad guys for rejecting you.
People who espouse this ideology are commonly known as incels and were brought up earlier in the thread. Incels follow a cult ideology (I'll explain this in a minute) and the way they are commonly talked about and talked to is not helpful if you actually want to prevent people from taking up that ideology or convincing people to leave it.
You have to understand just how attractive this idea is to someone dealing with rejection. It validates their existence and makes them feel better about themselves, it gives them a target for their negativity, and it gives them a support group of hundreds or even thousands of people who have experienced similar things. This is why I call it a cult ideology; cults grow in almost exactly the same way. If you want to know why the incel population has exploded so rapidly, just look at why cults are successful: it's because in many cases they're the only people willing to empathize with people in a crappy situation and offer them a way to improve it. Saying these kinds of groups prey on vulnerable people is true, but it also misses something important: that the people doing the "preying" are people who were in the same situation themselves and genuinely believe the cult has improved their lot. It's victims preying upon victims with nothing but good intentions.
This situation, which is already bad, is exacerbated by how people react to the "cultists." Typical reactions include disgust, mockery, scorn, etc. These are certainly natural reactions to someone saying some really stupid or horrible stuff, but they're also extremely unhelpful. Telling someone in this situation that they're awful, deplorable, hateful, etc. only further isolates them from normal society and drives them further into the arms of the cult, who tells them they're right and that the people condemning them are the bad guys.
So again, what to do? This problem is bigger than any individual person. Getting rid of the incel ideology requires large-scale changes in the way we collectively understand and respond to rejection and people going through it. It's not something that's going to be solved overnight, but if you genuinely want to help do away with it (and you absolutely should), I think you should start by being a bit less judgmental and a bit more empathetic toward people dealing with rejection. I'm not saying you shouldn't stand firm on your principles, because you should: people going through this do need to be aware of when they're starting to cross the line. But you should also understand that they're human beings going through something really awful and that what they really need is help getting through it.