this is the analogy I've always used to explain doping in cycling (and other endurance sports) to those who aren't into the sport, but it applies to all sports.
Let's say you worked all your life toward a single goal of gaining your dream job in sales. (I work in sales right now, so its the easiest analogy. everyone knows their dream job isn't in sales, but bear with me) Now let's pretend that your current employer is not only the highest paying sales job/organization on the planet, its really the only one out there other than a few small players that barely pay anything, if at all. If you lost your job, you really aren't sure what you'd do with your life. Now pretend that the entire sales force is incredibly competitive, and they'll regularly cut the bottom performers and bring in new people to fill the void. After a while, you find that you are slipping dangerously close to the bottom performers, and you have a wife and kids at home depending on your paycheck. Now you find out that 90% of the people around you are taking a pill that causes them to sell 25-50% more than you without them having to do a single thing, and there's very little chance they get caught. And even if they do, there's a good chance they'll just get a slap on the wrist. You probably have no or only partial education and no other work experience. What do you do?
obviously that mostly explains the middle of the pack types, not the ego-driven people like Lance, but it really makes people understand why doping is so rampant. Even those who might not normally dope can easily fall into it.