2) You’re bored of your program and just going through the motions.
Training programs are just like sex. If you keep doing the same position with your girl, she’ll get bored, just as you’ll get bored with doing the same exercises week in and week out. Your body will actually get better and more coordinated with the high repetition of exercises, but if you’re not progressing the weight used, decreasing the breaks between sets or changing how you perform each rep/set, your body will cease to adapt.
Follow the Principle of Progressive Overload, which states that to continue your body’s adaptation to exercise, the difficulty of the exercise must continually change. Also, change up your program before you get bored—it will keep things interesting but don’t be that guy who changes programs every two weeks. Also, try a phase where you lift heavy, say sets of 6 or less. You’ll be amazed at your improvements in strength and muscle-building potential.