I took up triathlon a while back - and I'm enjoying every moment. Why? Here are my reasons for choosing triathlon rather than, say, marathon running or underwater hang-gliding.

1. The Triathlon is a solo sport. You can train alone, compete alone, enjoy the participation without needing anyone else around. This, for me, is the main motivator.
2. It rearranges your life into a healthy pattern. You drink less, eat better, wake earlier and need less sleep. Your breathing gets easier and your heart pumps with less effort; even your thinking gets clearer. And all this happens as a natural aftereffect of training; it's not something you need to work at separately. Triathlon is the backbone of a healthier life.
3. You get a better body. Within a year your body will be one sleek sculpture of muscle resembling a Greek god. Shoulders a solid yoke, stomach a flat fulcrum, legs two pillars of enduring power. Your breathing will be even and your resting heartrate lower. (Now, if you could just eliminate the owl-eyes from goggles and the constant neck scars from wetsuit chafing...)
4. Your mind gets fitter, too. When you can deal with the difficulty of a Tri, your other problems get turned to a lower volume. You'll think clearer, faster, and be able to make better decisions. Stressful situations just don't bother you any more. You are truly a superior human being.
5. Triathlons are FUN. You don't get bored in a Tri; while you're swimming you can entertain yourself thrashing around with hundreds of other people, on the bike section you can relax and find a rhythm, and on the run you can test your stamina and smoothness. In between, there's the calculated concentration of the transitions. Outside of rock climbing or free-fall parachuting, it's as interesting as exercise gets.
6. Triathlon is a whole-body exercise programme. You use three different sets of muscles, and each keeps the other two in balance, giving you tone with far fewer of the sports injuries single-discipline athletes suffer.
7. It's a great sport for equipment fetishists. Three disciplines, and dedicated equipment for each. Tri wetsuits are totally different to normal ones, a Tri bike has steeper angles and a different posture, and Tri running gear means skin-tight one-piece trisuits you can swim in too and elastic laces to save time in the transitions.
8. It's easy to get started in Triathlon. Amateur triathletes are the friendliest and best humoured people in sport, combining the best of those great 'cameraderie' sports, cycling and swimming. Every Triathlon except the Ironman distances includes a big percentage of novices; whatever your level, you'll never be alone.
9. It makes you better in bed. 'Faster, harder, longer' doesn't just apply to the event itself. And why do you think they called one event the Ironman?

For anyone who wants to try a Tri, head for the British Triathlon Association or equivalent in your country. You'll never regret it.