"I'm not flexible enough for yoga" is what I hear from people all the time. I would tell them jokingly "if everyone is born flexible, I would be out of job!"
Some people are born flexible because they do not naturally hold on to any tension in their body. It also means that their muscles do not get as tight as others even as we move through our day to day activities.
Our body tenses up based on how we stand, walk, move or even think.
The way to free ourselves from muscular tightness, back, shoulder and neck pain is by watching yourself and having more awareness of your body and putting your muscles in a relaxed state when they are not suppose to be working.
That, to me is the practice of yoga. It is a practice to learn about yourself, your body and you!