That's always been a mystery to me too! You would think that doing nothing but sitting, eating and sleeping would make you feel great! Some ideas:
- Cabin air isn't very fresh, even on newer planes, especially after 5 or 6 hours on a packed flight. I'm sure the stale air contributes to feeling tired and sticky. You also can get very dehydrated, which makes you tired and sick.
- You're sitting on a seat that hundreds of people have sat on before you, and hasn't been washed in months. Sitting in a cramped space for a long time messes up your nicely pressed clothes, too, and can make your muscles sore and swollen. Also, if you're on a long trip, you probably haven't had an opportunity to really clean up (shower/shave) in a while! Even a short flight can take most of the day to complete your trip.
- "Standard" airline meals are notoriously unhealthy. Loading up on carbs and fat and just sitting there probably doesn't help, kind of like taking a nap after Thanksgiving dinner.
- You're already stressed, hot and tired from leaving the house, carrying your baggage through the airport, standing in lines, rushing to make connections, etc. Maybe you're worried about getting to your destination, too (unfamiliar city, lost bags, important meetings, etc.)
Just some ideas. All I know is, I try to bring an extra shirt and my toiletries kit on board so I can freshen up when I arrive.