The Torah describes Yaakov's encounter with Eisav: "And Eisav ran to greet him and he hugged him, and he fell on his neck and he kissed him and he cried" (Bereishis 33, 4). Rashi cites an opinion in Chazal that this was a true display of Eisav's emotions. "Rabbi Shimon bar Yochai said, 'It is a halacha that Eisav hates Yaakov. Nevertheless, his compassion was aroused at that time and he kissed him wholeheartedly."
Rav Wolbe (Shiurei Chumash) asks why Chazal referred to Eisav's hatred of Yaakov (i.e. the Jewish People) as a "halacha." Would it not it be more accurate to describe this phenomenon as agaddah? The answer is that agaddah and mussar are also halachos. Rav Avraham Grodzenski zt"l brought a beautiful "proof" to this idea. The Rif (one of the first Rishonim) wrote a sefer dedicated solely to summarizing all the halachos that are found in the Gemara. Yet, he included many agaddos in his sefer. The same can be said of the Rambam who wrote a similar type of sefer.
We are mistaken in classifying the mitzvos involving our hearts or relating to middos, strictly in the category of mussar. The above Rishonim understood that even agaddah and mussar fall under the banner of halacha. As a matter of a fact, the constant mitzvos such as loving, fearing and cleaving to Hashem, which are the very fundamentals of the Torah, all relate to the duties of our hearts.
Many people perceive mussar as extra credit. It's a nice thing to learn and live by, but it isn't as important as the studying and performance of the "real" mitzvos. It was due to this mindset that Rabbi Bachaya was prompted to write his renowned sefer, Chovos Ha'Levovos - Duties of the Heart. In his introduction he proves that the duties of the heart are just as obligatory as the duties of the body. Moreover, proper performance of the mitzvos involving the body is contingent on the proper performance of the mitzvos involving the heart! The Mishna Berurah writes that everyone is obligated to learn mussar every day. Someone who only had ten minutes a day to dedicate to learning once asked Rav Yisrael Salanter how that slot should be filled. Rav Yisrael answered that it should be spent studying mussar, and then he might realize that he really has more than ten minutes a day! We all know that we must invest time on a daily basis learning halachos. Shouldn't the agaddah and the chovos ha'levavos that pertain to us each day, and every part of the day, deserve at least as much consideration as those halachos that only apply once a week or once a year?