The Torah establishes different levels of responsibility for different types of guardians. A hired watcher — someone paid to safeguard another person's property — bears liability if the item is stolen or lost on his watch. A borrower — someone who takes an item for his own use — bears an even heavier burden: he is liable even if the item is destroyed by circumstances beyond his control, including the animal's death.
The Mekhilta constructs a kal vachomer, an argument from lesser to greater, that the rabbis considered one of the most powerful tools of legal reasoning. The logic runs as follows: A hired watcher is not liable if the animal in his care dies of natural causes. Death is beyond his control, and the law does not hold him responsible for it. Yet this same hired watcher is liable for theft and loss — lesser misfortunes that fall short of death.
Now consider the borrower. He is liable even for the animal's death. The Torah holds him to a higher standard than the hired watcher across the board. If the hired watcher — who escapes liability for death — must still pay for theft and loss, then surely the borrower — who cannot escape liability even for death — must also pay for theft and loss.
The conclusion is inescapable. The borrower's greater obligation in the more severe case (death) necessarily includes obligation in the less severe cases (theft and loss). The Mekhilta uses this reasoning to close a gap the Torah left open, ensuring that the legal framework remains internally consistent and that greater responsibility in one area cannot coexist with lesser responsibility in another.