Aveilus

בָּנִים אַתֶּם לַיהֹוָה אֱלֹהֵיכֶם לֹא תִתְגֹּדְדוּ וְלֹא תָשִׂימוּ קָרְחָה בֵּין עֵינֵיכֶם לָמֵת   

Ramban quotes Rav Avraham ibn Ezra as saying that this is an  issur to harm ourselves in mourning over a deceased close relative because we believe that everything that happens to us is a godly decree for our own good. We don't necessarily see how or why it is to our benefit but we must believe that God is all good. However, why then do we have  a mitzvah of Aveilus. It is a positive commandment to mourn a close relative. Mourning includes sitting on the floor rather than on a chair during the period of mourning. One is not allowed to bathe for pleasure or groom him/herself during the mourning period. Why then does the Torah mandate that we not cut ourselves or act out any of the other signs of mourning described in the Pasuk above?

Rashi in Tractate Succah 26A explains the Talmud's statement that one who is occupied in mourning is still obligated to perform other commandments. The Talmud states that although one who is occupied performing most commandments is exempt from other commandments, one who is occupied in mourning is occupied in a "Voluntary Occupation". Rashi explains that mouring is a "Voluntary Occupation" because "Although one who is mourning must sit on the floor and not bathe, he is not obligated to mourn per se but rather to show respect to the dead relative". 

Rashi is saying something very fundamental. Rashi is explaining that although one must mourn, the reason is not to feel obligated to feel sad but rather act sad. The Torah is not mandating an emotional response to a relative dying but rather mandating a show of respect to that relative. Respect is to show that I care about a close one and will not go out and party soonafter he dies but rather show that I experienced a loss and reflect on that person's life. 

Rav Avraham Ibn Ezra is taking this concept a bit further. Ibn Ezra is saying that the Torah wants you to respect that dead relative by contemplating his life but not allowing you to harm yourself in mourning which would show an unhealthy attitude towards God's decree. Cutting oneself is a sign of hurt that insinuates a greater disappointment in God's judgement that creates a negative attitude. We must show respect for God and respect for our loved one simultaneously by mourning in a halachic manner.