Many people think that if they are not sore after your workout its not been a good session and you are not effectively stimulating your muscles for growth. However how sore you are is not a good indicator of how effective your training is for muscle growth. (Have a look at this study if your interested).
Although muscle soreness may make you feel like you just had the best workout ever, it actually isn't a good indicator of how effective your training was for muscle growth or strength gain.
Muscle soreness usually occurs when you perform exercises you are not familiar with or are familiar with and have done some eccentric training.
The more often you perform a certain type of exercise, the less muscle damage and soreness it causes over time. This is known as the "Repeated Bout Effect" and it explains why you become less sore when you train consistently.
So, if you are making progress in the gym, you don't have to train for constantly sore muscles. In fact, being super sore all the time can inhibit you from pushing hard in the gym by increasing recovery demands and impairing performance. (See study here).