Why does doug need to adjust his workouts when they seem too easy?

Doug needs to change the exercises he is doing. Your body begins to become accustomed to what you do, and if you do the same thing every single time - you will stop making progress, and they'll simply become easy.

However, if you walk into the gym and your body is expecting you to just go over and bench press some, go rock some curls, head over to the squat rack then go home - but instead you start out with some dips, then go do lunges, then attack your triceps instead of biceps, your body will be shocked! 

When you change it up, your body will react. The goal is to be comfortable being uncomfortable. Also, one of the most overlooked things in working out is the intensity. If you bring it everything you got and go to the last gut wretching rep, you'll see results. 

However, if you just leisurely do things with 4 minute rests in between - you won't see true progress.

Hopefully, this helps! =)