tell me why ain't nothin but a heartache

The phrase "Ain't nothin' but a heartache" from the song "I Want It That Way" by the Backstreet Boys means that the situation described (most likely a troubled romantic relationship) is nothing more than a source of emotional pain or heartbreak. It expresses the sadness and disappointment felt because the relationship is flawed ("ain't nothin' but a mistake") and the singer does not want to hear the other person say "I want it that way," which can be interpreted as wanting the relationship to be a certain way that causes hurt or separation. The song overall is about a love strained by distance or emotional disconnect, where the singer laments that things have fallen apart and he doesn’t want to accept the heartbreak or the idea that the relationship cannot be fixed. The repeated "tell me why" phrases show a longing to understand why the love has gone wrong and the pain exists. Although the lyrics are sometimes considered confusing or not literally logical, the emotional core is about heartache and regret in a relationship.