1. A good chunk of the world’s population has been mysteriously raptured, and he’s upset.
2. There’s been a nuclear holocaust, and he’s the only survivor on a destroyer, or at an arctic outpost, or onboard an orbiting spaceship. So he feels grimy.
3. He’s just had, or is having sex, with one of the show’s female leads, whom he will soon betray. This will require a subsequent appearance wearing only a towel.
4. He’s working on a tough murder case at an urban precinct, and his female partner is about to surprise him with a late-night visit to discuss possible suspects. Which is why he’s only wearing boxers and looking exasperated.
5. He’s a corrupt CEO with his own penthouse with a private indoor pool, so just before the feds arrive to arrest him, or at the very least serve him with a subpoena, it’s time for a swim.
6. He needs to dive into the Hudson to save either a child or a crucial piece of evidence.
7. An uptight, older, secretly lecherous female supporting actress has just arrived, during his rigorous morning workout.
8. He’s kept his shirt on for the past two episodes, so his chest and abs are getting restless.
