さぞ

1. 1Imagines, with sympathy, how strongly a third party must feel in their present situation.Ex. 1
2. 2Addressed to the listener, it acknowledges how intense their past experience must have been.Ex. 2

Used when imagining the emotions or hardships of others, often in letters, speeches, sympathetic remarks, and polite conversation. Typical partners are feeling words like 不安, 緊張, 喜ぶ, つらい, and 大変. Unlike plain だろう, it adds both intensity and a tone of compassion or shared feeling toward the person described.

• に違いないN3「に違いない」 comes at the end of a sentence and expresses strong, well-founded conviction about anything, not just feelings. 「さぞ」 is an adverb paired with 「だろう」 and similar endings that empathizes with another person's feelings, situation, and their degree, with a sympathetic or exclamatory tone, so the two are not simply interchangeable.
• さもN1「さも」 describes someone's outward manner as looking a certain way, often with an ironic sense of putting it on, though it can also stress a clearly visible demeanor; it must be followed by 「かのように」「そうに」 or similar and leans literary. 「さぞ」 empathetically guesses at inner feelings and pairs with 「だろう」 and similar endings, so they are not interchangeable.
• に相違ないN2「に相違ない」 is sentence-final and expresses strong, well-founded conviction ("surely"), with a formal, stiff feel. 「さぞ」 is an adverb that empathetically guesses the degree of someone's feelings or situation, with 「だろう」 or similar at the end, so they are not interchangeable.
• だろうN5「だろう」 is a sentence-final conjecture form that can guess about anything, and with rising intonation it can also seek confirmation. 「さぞ」 is an adverb mostly used to sympathize with someone's feelings or situation, or to guess exclamatorily at the degree of a state, and it often pairs with 「だろう」 and similar endings; the two are not interchangeable.
• に決まっているN2「に決まっている」 is sentence-final and asserts "surely" based on common sense or subjective judgment, with a firm, colloquial tone. 「さぞ」 is an adverb used with 「だろう」 and similar endings to empathetically guess at someone's feelings or situation in a softer tone, so they are generally not interchangeable.
• どうやらN2「どうやら」 makes an objective inference from signs ("it appears"), with 「らしい」「ようだ」 or similar at the end, and also has a separate sense of "somehow, barely managing." 「さぞ」 sympathizes with someone's feelings and imagines how deep they are, pairing with 「だろう」, so they are not interchangeable.

に違いない, に相違ない and に決まっている are sentence-final expressions of firm certainty about a fact and lack さぞ's sympathetic imagining of feelings. Plain だろう is neutral conjecture that さぞ intensifies, どうやら infers from evidence ("apparently"), and さも describes an outward appearance ("as if"), not an imagined inner state.