Athletes who have an extensive training background usually have a good strength base, good conditioning base and a high degree of neuromuscular efficiency. They have the ability to put their body through so much stress (because they can lift heavy loads, do large volumes of conditioning, and sustain high power outputs), that they may not be able to recover before their next scheduled training session. If you haven't fully recovered before your next training session, you will never super compensate, and you will eventually enter a state called 'over-reaching', which can lead to overtraining syndrome (which is something you want to avoid at all costs!). Optimal development takes place when training stimulus is balanced carefully with recovery periods.
Let's look at a beginner now. Unlike the athlete above, the beginner hasn't spent hours upon hours in the gym. Instead, they have most likely spent hours upon hours on the couch watching television. They are typically de-conditioned, not very strong and they don't have the ability to activate all of their muscle (low neuromuscular efficiency). This means they can't place as much stress on their body; even if they wanted to! Sure they will get sore, but it will most likely be due to the exercise being a foreign stimulus - just like how an advanced trainee becomes sore after a new exercise is integrated into his training regimen.
Training hard for a beginner is critical. Habits are developed in the early stages of training. Beginners need to know they can push beyond what they think is their limit. They need to develop mental toughness because it trumps any other fitness parameter. It is a scary weapon, and you need to be equipped with it early. It will save you in your toughest workouts.
NOTE: We aren't saying train like a maniac the first day you enter the gym. Make sure before you add intensity to anything, the movement you are adding it to is technically perfect.