Most successful people have said that without passion success is very difficult (See how to find your passion).
When you do something you are passionate about the chances of giving up become lower, you become more able to go through hard times and so your chances of succeeding increase.
The pain of waking up early every morning to do something that you hate can ruin your life. Add to this feeling bad on weekends because you will have to do something that you don't like when you get back to work.
You need to have a lot of passion for what you do because it can get very difficult. Without passion, any rational person will probably give up (See how Steve Jobs started the Apple company).
If you are passionate about what you do, you will be more willing to work late nights, weekends and extra hours. This will increase your chances of success.
So many businesses fail because people introduce products that customers don't really need. When you have passion for something you will understand it perfectly and you will be able to figure out the market needs that could turn into business opportunities (See spotting market needs).
'Passion is energy. Feel the power that comes from focusing on what excites you.'
When you do something that you hate you won't find much energy or motivation. When you do something you are passionate about you will have enough energy to go through hard times and setbacks until you become successful.
Even if you succeed without following your passion you would still be doing something that you hate, everyday. Your life won't be a happy one.
We become passionate about the things that help us satisfy our important psychological needs. Thus following your passion won't just make you happy, but also psychologically stable.
When you do something that you dislike you will want to get it done as quickly as possible. This attitude closes the door for creativity and might prevent you from coming up with brilliant ideas (See how to become creative).
The success stories of many successful people have shown us that a passionate person can't be stopped because even when things go bad that person keeps going. The one who is not passionate on the other hand won't have a reason to keep going.