This bears repeating: Do you need to feel pain in order to gain more strength and muscle mass? The answer is NO. The pain is merely your body's signal that you trained hard a couple days ago and that's all. The pain is NOT a necessity for increased strength and muscle mass to develop. In fact, muscle soreness is not a good indicator of muscle damage. This means that muscle damage (and the inevitable healing that occurs afterward which causes muscle hypertrophy and increased strength) can occur irrespective of muscle pain. Experienced lifters become more aware of this over time as their body adapts to the training stimulus (i.e., loads, sets, reps, etc.) and they tend to not feel as much residual muscle pain. As a side note, be sure to understand the difference between muscle pain ("good" pain) and joint pain ("bad" pain). Generally, "good" pain should feel like a dull, slow-onset, aching pain that occurs during a hard workout set (i.e., muscle pain) whereas "bad" pain will feel like a sharp, quick-onset, focused pain (i.e., joint pain). Remember, pain is the body's response mechanism to a stimulus, whether good (muscle pain) or bad (joint pain).
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."