I think the loneliness does not come from the fact that you are only two people on board of the aircraft. In my opinion the different rosters make the difference.
With us the cargo rosters are much longer, and bring you to much more "remote" places than the passengers flights do.
i.e.: Pax flights go from homebase to your destination and one or two days later back home. So usually after 3 to 5 days you are back home.
Cargo roster may give you five different layovers resulting in rotations of up to 10 or 12 days. And, very important, the cargo schedule is not as stable as for the pax flights. Sometimes you come home a few days later, because there was not enough freight on the originally planned day.