Ok, I know the motto "no pain, no gain", but do you HAVE TO feel muscle soreness to get results? I like the feeling of soreness the next day, but some days I don't feel anything. Does it mean I did not work my muscles hard enough (because I do feel the burning when I perform my exercises) or do you have to change up your routine every week?
No you don't necessarily have to feel pain to be getting the benefits. Adjustments should be made periodically but, don't think this means you have to change exercises. There are four variables that can be manipulated - speed, intensity, rep range and exercises. Periodization training is very effictive and uses variations of these.
I often find it's helpful to "******" my questions to see what other people have written about out there on the web. There are usually good articles written by doctors not actually trying to sell you something that come right up. I came up with this as an answer. Per the rules, I don't think I can tell you the website (?) :
"After a hard workout, most people begin to experience soreness in the body parts trained within 24-48 hours, lasting for as long a week after the soreness sets in. This type of pain is called "Delayed onset muscle soreness" or DOMS.
Scientists aren't 100% sure of all the causes of DOMS, but they have a pretty good idea. The burn you feel during the performance of an exercise is caused by the buildup of lactic acid, a by-product of exercise metabolism. It was once thought that the next day soreness was a result of this lactic acid staying in the muscle. Today, most exercise physiologists agree that the primary cause of DOMS is the tiny tears that occur in the muscle that as a result of high intensity exercise - especially resistance training.
When you work out, you literally "tear down" muscle tissue (these are microscopic tears - not like a "torn" muscle in the medical sense). During the days after the workout, the muscle begins to rebuild itself, provided it is allowed enough time to recover and sufficient nutrients are provided. This rebuilding process creates a "new" muscle that is bigger and stronger than before. In a nutshell, this is how the enitre process of muscle growth takes place.
This type of pain is different than the burn you feel during the workout and it is different from the pain of an injury. It's important that you develop the ability to differentiate between the "good pain" of soreness and the "bad pain" of injury. Unless the soreness is so extreme that it is debilitating and prevents you from participating in sports or performing routine tasks (like walking up a flight of stairs!), then next day soreness is GOOD PAIN! It is a sign that you had a good workout - that you trained hard enough to break down muscle tissue. As a result, your reward is going to be bigger and stronger muscles. DOMS will be greatest in a beginner who has never worked out before. The more your body adapts to the workload you impose on it, the less soreness you will feel. If you continue to repeat the same workout over and over again, it will eventually cease to make you sore. Unfortunately, you will also cease to make any progress. Progressive overload is the cornerstone of getting stronger and building muscle. "
DOMS is not felt as often in heavy weight low rep workout however, this form of a workout has great benefit especially when used in periodization. I would agree with the above statement but it is tailored toward the very basic workouts.
Muscle Soreness 
