Sometimes life just isn't fair.
Consider two sisters, one who is a couch potato, and the other, an active tennis player.
While the tennis player ought to be fitter and healthier in every way, she opens herself up to the danger of sports related injuries like tennis elbow, which her couch potato sis will avoid.
Your elbow is much more complex than, say, the hinge of a door. Three separate bones, two from your forearm and one from your upper arm, come together at the elbow. In addition to these bones, your elbow is also the place where your wrist muscles are anchored by tendons. It's these wrist muscles that are the key to tennis elbow.
The muscles you use to bend your wrist backward are pretty delicate, so it's easy to strain them from a number of activities. If you strain them enough, these muscles and tendons will swell painfully along your whole forearm, with most of the pain concentrated around your elbow. You can get tennis elbow from playing golf, hammering a nail, or any other wrist-intensive activity.
If you've gotten tennis elbow from playing tennis, you probably hit a lot of backhand shots. When your backhand swing isn't textbook perfect, you use those delicate wrist muscles to whap the ball.
A gentle weight training program, to strengthen your wrist muscles, can help prevent tennis elbow.