Our Gemara on amud beis discusses what are the liabilities of a rented object.  A paid watchman is liable for ordinary theft that could have been prevented through vigilance, while an unpaid watchman is exempt, so long as he was not negligent.  The basic idea is that if you are paid, you are expected to devote more energy and focus on safeguarding the object.   However, a renter can be seen either way:  Is he like a paid watchman, and his “pay” is the the use of the rented object, or do we say that since he already pays a rental fee, that neutralizes the benefit of usage, and now it is as if he is a free watchman. The Gemara says:

With regard to a renter, whose legal status is not stated explicitly in the Torah, how does he pay in the event that a rented article is lost or stolen? Rabbi Meir says: He pays like an unpaid bailee, i.e., only in cases where the loss of the item was due to his negligence. Rabbi Yehuda says: He pays like a paid bailee, i.e., even in cases where the loss of the item was not due to his negligence. 

The Shalah brilliantly analyzes Torah observance and our contract with God in this light (Aseres HaDibros Pesachim Matzah Ashirah).  He explains:

Each of the four watchmen represent a different way of relating to God, similar to the Four Sons of the Haggadah.  The Free Watchman serves God expecting no reward, and therefore is not held liable for certain sins that he commits due to being overtaken (“stolen”) by his impulses; he is only liable for sins of outright negligence. The Hired Watchman represents the person who serves God expecting a reward.  Therefore, he is held liable even for sins that are committed impulsively or compulsively, but still exempt for sins committed that could not have been foreseen or prevented.  The Borrower represents the person who really does not serve God, he does not believe in reward and punishment, and he takes and takes.  Because of that, he is held liable even for sins committed that he could not prevent or stop.  

The Renter represents someone who serves God generally without expectation for reward, but may make a particular mitzvah conditional, such as a person who says, “I am giving this donation to charity on the condition that my son is healed.”  Shalah quotes the Semag who rules that this is permitted, when the person resolves to be happy having given the charity regardless of whether his son is healed; he is merely dedicating the mitzvah as a merit.  However, if his attitude is that “God owes him”, then it is forbidden.  This why the liability of the Renter is represented in the Gemara as an opinion that is variable and fluctuating because the person who performs a particular mitzvah with the hope of a specific merit and benefit (“the Renter”) may be hardly liable, as the Free Watchman, or heavily liable as Borrower, depending on his attitude. (In the mystical world there is no such thing as machlokes, but instead just different dimensions of truth, see Psychology of the Daf Eiruvin 45.) If he acts with a full heart and with only hopes, but no demands, then he is treated by God as a free watchman and is less liable for sin. But if his observance of mitzvos is utterly contingent on receiving a reward, then in a self-fulfilling manner, he is now engaging with God as a paid watchman, with more liabilities for sin.