1. they are treated unfairly
2. they normally live in terrible conditions
3. performing tricks is actually bad for their health.
4. It breaks up the elephant families.
5. elephants don't get the EXERCISE they need IN THE CIRCUS.
6. they are incredibly INTELLIGENT and emotional animals that shouldn't be kept in circuses.
"Elephants have one of the most complex social structures of any species -- and more is being learned every day. They are capable of deep and intricate emotions. [...] Elephants are highly intelligent, emotional animals who don't belong in circuses, where they live in cramped conditions and are separated from their families."