I couldn't really place it, even with therapy, until I came across a quote by Carl Gustav Jung:
"Loneliness does not come from having no people about one, but from being unable to communicate the things that seem important to oneself, or from holding certain views which others find inadmissible."
This instantly rang very true for me. Whenever I feel lonely now, I think about what is important to me, and then identify or find "the right people" to talk to about it. Over the past few months, it really helped me more than anything else.