Stop lying to yourself. Nobody is a complete lifter.
1) Working Weaker Muscle Groups Builds Strength And Improves Overall Body Composition
The muscle groups that are least worked over time tend to have the potential to grow bigger. While the muscles you work regularly are more likely to hit a growth plateau, under worked muscles will take the stress and push towards growth. Once the underworked muscles have grown and gained considerable strength, you will notice a better overall balance in your overall body composition.
2) Doing The Lifts That Scare You Is A Clear Sign That You Are Maturing As A Lifter
Doing what scares you is a sign of a lifter who has deserted his ego and understands the science behind getting bigger. Real strength lies in doing the lifts you have ignored for the longest time. If you are better at upper body strength work, maybe it’s time to focus more on squatting. You can’t be best at all lifts but you can surely be better than you were.
3) You Will Fail To Eventually Succeed
Chances are that you will fail while doing new lifts. You’ll fail and fail and fail again. Right at this stage you need to realize that only doing the lift will make you feel accomplished, not quitting. Newer exercises will challenge you. The undertrained muscles will pain. But if you are relentless, you will prevail.
4) You Will Know How Your Future Training Should Look Like
Doing the same thing every day is easy. Doing something you have never done before will kick your ass. Learn from failure and use it as an opportunity to program your future workout. That’s how life-long lifters proceed.
5) Most Importantly, You Will Learn How To Lift Out Of Your Comfort Zone
You can squat 60kgs on the back squat. Great! But can you do that in your front squat? You will never grow until you shock your muscles and push it out of the comfort zone. Failing at certain lifts will teach you just that.