You will face it, too

If you are an athlete, you will probably face tendonitis at some point during your playing days. If you're a regular human being, you'll most certainly face it. Really common areas that are affected include the wrist, the elbow (tennis or golfer's elbow), the shoulder (rotator cuff tendonitis), the hip, the knee (patellar tendonitis), the heel (Achilles tendonitis), and the ankle and foot.
When you break it down scientifically, you see that joints move by the power of muscles. Muscles attach to bones and joints by way of tendons. These tendons transfer the force from muscles across the joint and cause it to move. Tendonitis happens for a variety of reasons. When the normal smooth gliding motion of a tendon is impaired, it can become inflamed and movement in that joint becomes painful.
The most common cause of tendonitis -- by far -- is overuse. When you perform repetitious activities, increase the intensity level of an exercise program, or simply "overdo it," the tendon sometimes responds to the new level of demand by developing inflammation and thus, tendonitis.
Another common cause is due to age-related changes of the tendon. As the tendons age, they lose their elasticity and ability to glide. We are still doing research to better understand the causes of these changes, but hypothesize that the cause may lie in the fact that blood vessels supplying nutrition to the tendons change and decrease in effectiveness over time.
A tendon can also become inflamed or injured abruptly. An injury can range from a simple strain to a rupture or tear (this is generally when orthopedic surgeons such as myself are really needed to put the tendon back where it goes). Common tendons that can rupture include the rotator cuff, the biceps tendon, the patellar tendon, and the Achilles tendon.
Treatment of tendonitis tends to be conservative. Most physicians will recommend some simple treatments like rest, activity modification, bracing, anti-inflammatory medicine, and sometimes injections. Physical therapy and the appropriate rehab can also help you "mend" faster, as well as strengthen your muscles and improve flexibility to prevent recurrent flare-ups.