Pandas fell into a very weird evolutionary trap. They should all already be extinct by now, but they barely keep making it. Tons of fast growing bamboo all year round, no predators, and a little evolved fake thumb that let's them hold a piece of bamboo. They just stay within like 50 feet of their "beds", eat like 80 pounds of bamboo a day, and go to sleep.
The tons of eating and high fiber poops don't sound so bad, but even if I really like "burritos", every now and again I'd crave some ribs or something.
They have special body chemicals so the food gets them high. Basically if your doctor said you could only eat weed infused food or your stomach can't handle it. What a hard life...