For an "Other"

The Baal HaSulam discusses the idea that there is greater value to Mitzvoth that are done to benefit our fellow man (בין אדם לחבירו), as compared to Mitzvoth that are designed only to serve God (בין אדם למקום). He mentions five distinctions:

1. The Mitzvoth that are serving your fellow man are not set, but are always changing based on the specific current needs of the recipient of your kindness. There are always new challenges that need to be reckoned with and overcome. Therefore, it is impossible to do them by rote.
   
2. When ones is performing a Mitzvah directed only to Hashem there is no expectation of any immediate response; you don't hang around waiting for a "thank you." When assisting another there is a temptation to expect a quid pro quo. Learning not to expect anything in return can generate great personal growth.
3. Helping someone else shows a recognition of an "other." Creation was facilitated by God in order that there be an "other" to whom God could do good. One of the pre-requisites for receiving that good is the recognition of separateness from God; the recognition of oneself as distinct from God. A child sees the whole world as there to serve him and fails to recognize the distinction between self and other. With maturity there (hopefully) comes a recognition of self. The ability to see the needs of another person shows that that recognition has been achieved.
4. It is only through actively loving another that one can learn how to love God.
5. By serving another one is not only the recipient of God's kindness but the bestower of it as well. To only receive is not Godlike, to receive and pass it on makes one Godly.