Studies have found proteolytic enzymes help short circuit many pain pathways. They help reduce inflammation and swelling as well as assist in toxin and waste removal from your blood. Pineapple contains proteolytic enzymes such as bromelain. There is evidence that suggests that these enzymes have anti-inflammatory properties and may well help reduce pain and stiffness associated with arthritis much like non steroidal anti-inflammatories (NSAIDS) but without the dangers associated with long term use of NSAIDS.
Note there are some drug/nutrient interactions with bromelain increasing the action of warfarin and other blood thinners, it also increases the absorption of most antibiotics.