Your attitude defines and delimits your level of success. If you have a generally lousy attitude, you'll never really be successful. If you have a generally upbeat attitude, however, you'll always achieve at least some level of success.
Your attitude determines how you interpret what events mean. Take obstacles, for example. Everyone who pursues any goal will inevitably run into obstacles. Your attitude determines what those obstacles mean and therefore how well you deal with them.
If you have a lousy attitude, you'll see obstacles as threats and annoyances. Even if you overcome them, you'll find the process frustrating, which will make future obstacles harder to cope with.
If you've got a positive attitude, you'll see obstacles as interesting or even fun. Even if you fail to overcome them, you'll find the process invigorating, which will make future obstacles easier to overcome.
I ran into (or, more actually, almost ran into) a perfect example of this the other day. I was trying to find a parking space in a covered car lot. Suddenly, another car zipped around a blind corner and we would have crashed if we both hadn't quickly braked.
My first thought was: "Wow, I'm sure glad that we (meaning me and the other driver) have fast reflexes!" I'm not exactly sure what the other driver thought, but she gave me the single-finger salute accompanied by an expletive.
Same event, different attitude.
Now, it's never pleasant to be the target of that kind of animosity, but my next thought was to feel sorry for her. She was in a small car with four kids and clearly having a horrible day. (I felt sorry for the kids, too.)
Clearly still furious, she drove off even faster than she'd been driving before. I, however, slowed down because I took the almost-accident as a warning that I needed to pay more attention to my driving.
Same event, different attitude, opposite outcome.
The outcome for the other driver was an action that made her drive more dangerously and thus increase the possibility that she'd actually have a fender-bender next time. My outcome was to make me a better driver.
Everything in business is exactly like this. Stuff happens. Your attitude determines how you interpret and experience those events, and therefore strategies and tactics you'll follow in the future.
This is not to say that people who suffer from lousy attitudes don't win sometimes. They do, but it's always with massive hassle and complaint. When they finally win, they feel exhausted.
Similarly, having an upbeat attitude doesn't bulletproof you against failure. You'll fail sometimes, but your attitude makes it easy to learn from your mistakes. And when you do win, you feel exhilarated.
Some people believe that their lousy attitude is part of who they are. Not so. As I explain in "How to Create a Positive Attitude," your attitude is something that you're doing. It's not a character trait.
Furthermore, there are at least 8 ways to improve your attitude and 17 ways to be happier at work. I know these methods work from personal experience. I'm a bit embarrassed to admit it, but 20 years ago, I would have been the driver doing the salute.
Regardless of what you do for a living, your real job--the foundation of what you will or won't accomplish--is your attitude. The more upbeat you are, the more likely you are to be successful at whatever you do.