I think there's a pretty big difference between enduring loneliness and making a choice to be alone for some reason. Loneliness is a state of despair, brought on by feelings that no one wants to be around you or to be your friend, or that no one cares about you. It can be a seriously demoralizing state of mind and likely is the reason behind many suicides. Loneliness is, for the affected person, an almost constant feeling of inferiority for which there seems to be no way out.
Choosing to be alone would be a choice made by the scientist who wanted to devote time to some research, or it could be someone who's interacted for so long with so many other people, that being alone for once, is refreshing and invigorating. For a person choosing to be alone, they would not experience loneliness, unless they started to miss the interactions they had gotten away from. A person who chooses to be alone always has the capacity to change back to choosing to again have companionship. They also aren't ever sad because they are alone, as it is a choice rather than a sentence.