First, a travel backwards through time, please bear with me... Once upon a time, we lived in caves, hunted wild animals, spent time digging up roots and berries, and weathered extreme temperatures. Life was tough, and we dealt with it successfully. We were only motivated to really "tense" up, when there was a real threat: a dangerous animal was near, or there was an impending attack from another tribe.
So, our muscles served us well to keep us fed and warm and safe, and were only "motivated" to tense to extreme measures under a very REAL threat. Our bodies were in good shape from all the work we did just to survive.
Flashforward to today: We don't have to hunt or dig for food, we go to the supermarket. Our caves are lush and warm, with all the comforts you could ever need. Life is much easier, physically, and for the most part, we deal with it successfully. However, where we used to tense up against real threats, now we tense up against perceived threats. There are no tigers and bears, and in most of the world, we are relatively safe in our environment. When there was the threat of a tiger, we would tense, the tiger would walk away, we would relax. Now, if there is the threat of a paperwork deadline or a traffic jam, we tense, but we don't relax!!
This is why we need to stretch. Hands are tight from using keyboards, backs are sore from sitting in cars, and the one activity that does the opposite to those activities is STRETCHING.
If you get into a regular routine of stretching, you are undoing one of the stresses of modern life: tight muscles as a result of the condition we all live in.