"Lonely in the crowd" refers to the experience of feeling isolated and disconnected from others despite being physically surrounded by people. It's a feeling of not belonging, not being understood, or not being able to connect with those around you, even in a social setting. Link
This is an irony!
And quite often, it happens in the real world. I have seen people with a lot of friends, yet they are alone and they yearn for companionship.
A person can be in the midst of a crowd and still be lonely because he will be left alone since he does not know anyone in the crowd. That is why making new friends is very important, you might not be in a place where your old friends are.
This feeling of loneliness is not a good one because it can lead to other negative attitude too. So, you have to be careful.
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