One of the central obligations on Purim is the mitsvah of mishloah manot. Given that most mitsvot require a berakhah prior to their performance (Pesahim 7b, Megilah 21b, etc.) it is striking that no berakhah is recited on the performance of this mitsvah.  Many reasons have been suggested to explain why this is the case.  Rabbi Yehiel Yaakov Weinberg (Seridei Esh 1:61) proposes two unique answers that shed a new light on the nature of this obligation, as well as provide perspective on interpersonal obligations in general. 

He first suggests an answer that relates to the intent one must have when fulfilling the mitsvah of mishloah manot.  He claims that a person should give the gift because they genuinely want to express their love for the person they are giving it to.  Giving a gift out of obligation misses the point.  Saying a berakhah implies that one is only doing the mitsvah because God has commanded it, or with regard to rabbinic obligations, He has commanded people to listen to the halakhic authorities who have in turn created the obligation (Shabbat 23a).  While it is true that mishloah manot is a formal obligation, the core of the mitsvah is the establishment of good will among peers.  Friendship is not something that should exist only because it is commanded.

He then suggests an answer based on Rabbi Yitshak b. Mosheh of Vienna in his Or Zarua (1:140).  The Or Zarua explains that a berakhah is not required for mitsvot that are not limited to a specific time frame.  If a mitsvah can be done at any time, it does not require a berakhah.  With this, R. Weinberg suggests that the obligation of mishloah manot applies year-round.  Seemingly, he means that the obligation to maintain good will is constant.  Thus, the principle behind the mitsvah is constant, though the particular expression is only required once a year.  He compares it to the obligation to hear Parashat Zakhor once a year.  Even though the specific obligation of hearing the parashah applies only once a year, the purpose of that obligation is to ensure that a person will remember the message the rest of the year.  Similarly, Hazal only enacted the obligation of mishloah manot on Purim, but the purpose of that obligation is to remind us that we should actively promote camaraderie constantly.  Thus, he explains, there is no obligation to recite a berakhah.

The two answers of Rabbi Weinberg present a novel understanding of the purpose of mishloah manot.  One day a year there is an obligation to give gifts wholeheartedly in order to create positive relationships with our friends, family, and community.  The purpose of this yearly obligation is to remind and inspire us to work on our relationships throughout the rest of the year.  Mishloah manot should create a genuine feeling of good will that lasts.  To objectify and formalize this mitsvah with a berakhah, making it something we do only because of obligation would minimize its importance and defeat its purpose.