weight is a measurable quantity.
Visual identification of weight as a number, gives us an expectation of what that weight should feel like. It provides an easily trackable constant among many variables, making numbers an alluring point of fixation.
how can that 15kg feel like nothing one day and twice as much the next?
Our eyes (brain) can see and interpret numbers, but our body can't see, it only feels. This creates a big discrepancy between what we are seeing and what we are feeling/doing and since numbers are easy to track, we focus on them.. (eyes bigger than your stomach)? Ever wonder why? Visual dominance, negates the need to feel, and at most, feeling comes as a reaction instead of a consistent normal.
Lifting additional weight such as a kettlebell, provides an external source of information to the brain. If we are wanting to make change in our body, we must track and progress according to the internal- how it feels, how it moves that external source. The weight lifted (or any work done) must be relative to the nervous system. Ultimately, the goal is to increase the amount of stress that the nervous system can handle. Most people tend to approach this by blasting the system with whatever they feel like doing for the day and hope for the adaptations they want. But, if the nervous system doesn't think it's under attack and doesn't have to waste energy dealing with threats, it opens a very interesting door of opportunity..