No one wants to eat the same food every day, or wear the same clothes. So why would you always do the same workout? Variety is important in everything, including exercise. The two things you need to vary are...
1. Type: Every sport works specific muscle groups more than others, so if you only do one thing, you will develop muscle imbalances. There is the principle of specificity which says that the best way to get good at something is to do that thing as much as possible, but that has it's limits. Even professional runners who spend most of their workout time running will also work weight lifting into their schedule to keep everything strong.
And those of us who aren't trying to become world-class at one specific thing should probably have even more variety; if your main thing is a hard game of basketball with your friends, try swimming one or two days a week. If you always jog first thing in the morning, replace some of those with some weight-lifting. The key is trying to pick something as different as possible from what you normally do, so that you can recruit different muscle groups. Biking and riding an elliptical are pretty similar, so those wouldn't be the best contrast.
2. Difficulty: Most people know you need some hard days and some easy days. But I think the need goes more than that: you should do workouts that cover the complete spectrum of difficulty. And difficulty has two parts, duration and intensity, so make sure to experiment with both of those. When I was 18 and training for a marathon, every run was 45 minutes as hard as I could possibly go. And it was disastrous: I burned out hard after two months and needed some serious rest to recover. What I should have done is some 45 minute tempo runs at high intensity, but also some easy days with a light 20 minute jog. And I should have done some 60-70 minute runs at low intensity, and some hard, short sprint workouts.
One reason variance is so important is that your body is forced to adapt when it experiences something new; so if you always do the same thing, you wont grow. So change things as much as possible, and see what happens! Another purpose of variance is psychological: new things are more fun. Instead of a one trick pony, wouldn't you rather be someone who runs and bikes and swims and kayaks and spelunks? That sounds way more exciting. And guess what, the more excited you are to work out, the more of it you will do.