Why is knee rehab painful?

Inflammation. When a jont is injured, the lining of the joing is inflammed and the tendons, ligaments and muscles also get stressed and inflammed. When restoration of function is attempted and motion is induced and stretching occurs, there is activation of pain fibers. That is why a therapist is needed to guide you through this. Medications and pain alleviating measures may be needed to accomplish end goal.
No pain no gain. Unfortunately, sometime you must work through the pain to regain mobility and function of an injured joint. Your physician can give you mediation to ease your discomfort.