What are the most common causes of sore knee?

Depends. It depends on your age and level of activity. As you get older, arthritis is a common cause of knee pain while in younger population, trauma/strain remains a leading cause of knee pain.
Patellar pain. Soreness around the kneecap (patellar pain), is the most common cause of soreness at the knee joint. There can also be tendinitis above or below the kneecap.