The basic rule is that short, simple sentences–even sentence fragments–convey more energy than longer, more complex sentences. They are less likely to be beautiful in the poetic sense, but they carry more punch.
Take an emotion such as anger. If it is a long-held, smoldering anger, longer sentences with modifiers and clauses might be appropriate in a narrative passage. But if it is an anger outburst or a heated argument, you are more likely to want short sentences.
If you use lots of ands, buts, whens, and thens, consider whether wordiness is sapping energy from your writing. Consider breaking one long sentence into two or more shorter ones.