Being alone and being lonely are two different things.
Being alone is a phenomenon in which you are not in physical or social contact with or in close proximity to people.
But being lonely?....

You may be surrounded by people and still be very lonely on the inside.
You may be a social icon,  a celebrity,  a man/woman of the people, a jolly good fellow... And still be very lonely.

You may have good and jovial friends, but none of them to fill your emotional void.
You may constellate a host of social media fans, followers and admirers but none of them to truly pour out your heart and soul to.

You may be in a noisy crowd, but your heart is empty.
Being lonely is having no one to wholeheartedly confide in,  none to truly share your life with, to establish a meaningful physical, psychological and sexual connection with...on a very intimate and personal level.

Every human was created with an emotional void, delineated in the missing rib of Adam. We all have spaces in our thoughts, minds, hearts, feelings that should be filled by a special person.
And we are lonely when there is none to fill these spaces.

In fact its even possible to be in a relationship and still be lonely.
Its even possible to be married and still very lonely.
When the person you're in a relationship with or married to can't fill your emotional void, when you can't confide in them, when you don't share a meaningful and deep connection, when he/she doesn't give you adequate attention, you discover that you're still lonely even with someone by your side.