Exercise is one form of controlled metabolic stress.
And like all controlled stressors, it should be applied in a fashion that is stressful enough to sound the alarm but not too stressful to break your body.
However, in competitive athletics, this process becomes much more of a balancing act than one may expect. .
In life, there are many other forms of controlled and uncontrolled metabolic stressors influencing the body such as:
Lack of sleep
Internal perceptions and thoughts
Injuries and illnesses.
Now, these uncontrolled metabolic stressors may not be noticeable as we go through our daily lives, but once they begin accumulate, they can manifest in many different forms.
This is the whole idea of the human body acting as a dynamic system - it can be affected by various inputs in a variety of ways.
For Example, say you eat like crap, don't sleep well enough and train really hard.
This is hardly an ideal position to put your body in for any amount of time, let alone weeks or months at a time.
We have all probably heard that short bouts of intense exercise can put you in a state of "immunosuppression" and causes an "open window" period that can make you more susceptible to viruses. However, this has been shown to not be completely true - if we allow our bodies to recover and heal from the stress placed upon it and we take into account the other stressors (other than exercise) that we place upon our bodies.
When our dynamic system is functioning well and the body is able to recover from the various stressors placed upon it, immune defenses will actually acutely rise. Thus, acting as an immediate positive.
So just remember that lifting is much more than reps and sets. It's about how those reps and sets are fitting into the bigger picture.