Many times, when we stretch, our body is resistant. The muscles that have tightened from daily repetition of activities does not want to cooperate. If it did willingly, we would all be circus performers, Olympic gymnasts or yoga instructors. So often the body says "But I don't want to!" in regards to stretching.
That's ok. As my Teacher once (maybe twice!) told me, if there is something you don't want to do, that is reason enough to do it. Obviously, in the case of a tight muscle, its tight for a reason, and if it does not hurt already, it is going to. So, taking the "bull by the horns" and insisting on stretching when your body says "no way", you go a long way to undoing what life has a tendency to do to human bodies.
Now insisting on stretching does not mean vigorously forcing a muscle to do what it can't. It means going about your stretching program anyway, with great attention to detail (see howtostretch.com) and seeing your results in time. You will be glad you did.