Researchers have quantified what makes songs succeed:

• Optimal complexity — complex enough to stay interesting, simple enough to remember on the first listen
• Repetition — hits repeat their title many times, and familiarity breeds preference (the mere exposure effect)
• Surprise within structure — a predictable verse-chorus pattern with one unexpected element, like a beat drop or key change
• Simple language — the lyrics of a #1 hit average a third-grade reading level, far simpler than album tracks

Songs are also getting shorter. Streaming counts a play after roughly 30 seconds, so writers front-load the good parts and trim the rest.