Muscle tension is controlled by the central nervous system (CNS for short) mostly. This means that any activity that improves central nervous system function can contribute to increased flexibility. For example, doing pelvic mobilizations can improve hamstring and piriformis flexibility immediately. The same goes for mobilizing the neck. Immediate improvements can be seen elsewhere in the body. This of course, provided that these areas are "stuck" and need to be mobilized.
Riding along with concept that muscle tension is CNS dependent, it can be very easy to understand that mental / emotional stress impacts flexibility dramatically. I have noticed that there are 3 types of people when it comes to flexibility: First, those who are "gumby", meaning that they have a tremendous amount of joint flexibility and what appears to be little muscle tone. These individuals are much more likely to get joint injuries, and have "delayed" muscle activity.
The second type, those who are "bound" meaning that they have a LOT of muscle tonus, and are generally very "tight". These individuals are much more likely to have muscle strains, or tears, because the muscles are "hyper". The 3rd type, sits right in the middle of these 2, and shares characteristics of both.
The interesting thing about these types is that their personalities seem to reflect the amount of flexibility they have in their bodies. The more "gumby" the body, the more flexible they tend to be mentally, easily taking on new concepts and open to new learnings and ideas. On the other end, the "tight" people tend to have more "boundaries", and are less likely to take on new ideas, and be less "flexible" without a thorough filtering process before they change.
Now, I am not saying that stretching is completely useless, but instead needs to be assigned based on knowledge of the type of back pain sufferer that is seeking relief. As you may have guessed, the "bound" type of people will benefit MUCH more from stretching, and also more relaxation training. The "gumby" people will benefit more from strength and stability training, along with possibly reinforcing or setting up new boundaries in their lifestyles.