We use the two interchangeably because we’ve been conditioned to think of them as the same state; someone who spends time alone must, naturally, be lonely.
But that’s not the case.
Loneliness is being alone — and not liking it. It’s a feeling.
Solitude is being alone — and content. It’s a choice.
This brings us to an interesting paradox: if you can master solitude, you’ll never feel lonely again.