Did you know that loneliness can physically hurt? No, literally: "The same areas in the brain light up when you experience social pain as when you experience actual physical pain." That's how much loneliness affects us.
Humans, by design, are social beings. Whether we admit it or not, we can't deny the fact that our existence depends on interactions. Even introverts need some level of human contact, even though they are more comfortable with being alone most of the time. But this just proves that being alone is not the same as being lonely.
Though it's different for each person, loneliness is when you feel like you aren't connected to other people, or when you feel like you have fewer social connections than you'd like.
But according to psychologist Maike Luhmann, loneliness isn't all bad: "As long as we then do what we should do—reconnect with people—then loneliness is a good thing. It signals that we need to do something about our social connections. This is a sign from our psychological systems that there’s something off."