Absolutely, Indigo-
My DF & I are both only children. He has made me realize the truth of the lonely only theory- children don't need siblings, they need socialization. I was raised in the country with no neighbors. I was sent to small schools and rarely had company. DF came from a large extended family and went to public schools with many classmates.
I am envious of his ease around people. It benefits him in many ways and has made him an asset for his employers. It has been a struggle for me to learn social skills that I missed out on by being isolated as a child.
How you're raised makes all the difference.