Alone vs. lonely

If you're alone, it means there is no one physically with you, or no one is sharing your activity. You could take a trip alone, or do a project alone.

If you're lonely, it means that you feel a little sad to be alone. You wish someone were with you
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Do you like to be alone?
Do you ever feel lonely?

Some people say that the worst loneliness (this is the noun form of the adjective lonely) is when you feel lonely when you're with other people. The concept here is that even though others are with you, you feel isolated from them, without a strong connection. Have you ever experienced this?

When you look at this image, what is your impression: is he lonely, or just alone?