What is shelichus or agency at its core?  On the one hand, it makes sense that we should be able to appoint another person as an agent to represent us and execute transactions. On the other hand, it is absurd. How can anyone ever really speak for us and stand in our place?  Even more puzzling, how is it that certain mitzvos can be accomplished by agency, and others not?  

(Generally, the rule of thumb is that a mitzvah that must be accomplished by your own body cannot be delegated - “mitzvos she-be-gufo” - unlike some mitzvos that just need to be fulfilled.  You need a mezuzah on your house, so a shaliach can do it.  But you need tefillin on YOUR head and YOUR arm, that is your body, so no one else can wear tefillin for you. In terms of inyanei de-yoma, you can check out a Chasam Sofer and Torah Temimah that discuss this in terms of the mitzvah of Lulav and Esrog. See Torah Temimah Vayikra 23:40 and Teshuva Chasam Sofer OC 182:4.)  

Our Gemara on amud beis discusses situations where a person can revoke his appointment of a shaliach when the shaliach violates the mandate such as selling a property for lower than market price.  This is true, even if in the identical circumstance, had the owner made the same mistake the sale would still be valid. (Due to the principle of variability in real estate, the laws of overcharging or undercharging are less applicable, subject to much discussion in halacha.  But in regard to the agent, the owner can say to his agent you were not appointed to cause me a loss and invalidate the sale.)

Likkutei Halachos (Choshen Mishpat, Shelichus 1:1) explains the power of agency and shelichus from a metaphysical perspective.

It is known that all Israel is one soul, as it is written (Bereishis 46:27 ), "Every soul that comes from the house of Jacob was seventy souls." Soul is stated the singular. This is how one Jew can become an agent for another, as their souls are linked.  This is also why a gentile cannot be seen as a halakhically valid agent of a Jewish person.  A minor also cannot be an agent, because to be an agent requires a sense of the unity amongst the people, that requires a developed intellect.  A child cannot grasp this yet.  It is also for this reason that the shalichus becomes invalidated if the messenger violates the intent of the sender; he has broken the sense of unity and ceases to hold that connection to the other person.

A successful relationship requires an acknowledgment of mutuality.  Mutuality is not merger.  Merger is a painful boundary crossing that induces a sense of being utterly responsible for how the other feels, for their happiness and or misery.  That is not healthy.  At the same time, there should be deep concern for a partner’s (marriage, business or family member’s) well-being.  Your success and the other person’s success, your happiness and the other person’s happiness are not identical, but they are not disconnected. The key is to take full account and empathy toward the other person’s needs, even when they are contrary to your own, but not in exclusion to your own.

While this seems paradoxical, consider this metaphor. If you have a headache and need to lie down, but you also have a time-pressing matter that you must take care of or you will lose out, what do you do?  You don’t beat yourself up or intimidate yourself (that is, if you have healthy ego function!)  Rather you decide, all things considered what is best for you as a totality.  So one should be in relation to the needs of a loved one.  It is not about who wins. It is about taking all needs seriously and then seeing what is best.