If all of your friends are smokers, than chances are high that you will also smoke. Of course, if you understand the power of influence, then you can use this to your advantage by looking to associate yourself with people of the qualities that you're looking to attain.
“Get a millionaire mentor. Most of us were brought up middle class or poor and then hold ourselves to the limits and ideas of that group. I have been studying millionaires to duplicate what they did. Get your own personal millionaire mentor and study them. Most rich people are extremely generous with their knowledge and their resources.”
-Yvon Chouinard
My mentor also told me that he was able to determined my income based on the average income of my 5 closest friends. We can't deny that we're likely more easily influenced than we like to believe that we are. Be careful who you hangout with as you may end up like them.