Said the student to his Rebbe: “I want happiness”. Replied the Rebbe: “First remove the “I” which is haughtiness. Then remove the “want” which is desire. After that, all that will be left is “happiness”.
The Pasuk says (Tehilim 86:4): שמח נפש עבדך כי אליך אדני נפשי אשא - “Give joy to your servant, for to you G-d, do I lift up my soul. The Beis Aharon of Karlin explains that “lifting up his soul” to G-d refers to messiras nefesh. When we give over our will to G-d, we are essentially letting go of the ability / illusion of being able to control our own happiness in any way. But no human can live without at least some happiness or motivation. That is why Dovid Hamelech asks G-d, please give me joy G-d, for I am totally yours with complete abandon. When a person let’s go of the self, they are basically saying to G-d, "I will no longer attempt in any way to make myself happy. I throw myself into Your arms, and You G-d, will care for me and provide me with the joy and motivation necessary to live and be of service to You." When we do this, G-d Himself provides the happiness. And when G-d provides happiness, you can imagine that it will be a much greater and truer happiness than when we attempt to bring it upon ourselves by running after our own petty desires.
What indeed is messiras nefesh? Many people think it means throwing oneself into a fire ‘al Kiddush G-d’. This is true as well, but there is a common misunderstanding regarding this concept. After all, there are many terrorists who are willing to detonate suicide belts and kill themselves for “Allah”. What differentiates them from a Jew who is ready to die for G-d? The answer is simple, yet profound. Messiras Nefesh has nothing to do with what you are willing to sacrifice, even if it is your very life. As long as you are sacrificing it ultimately for your own self, it is a selfish act. When a terrorist blows himself up, he believes that he will go straight to Heaven and be given 72 virgins. He is actually doing the most selfish act possible - that of killing other human beings for what he imagines is his own personal ticket to eternal bliss. This is the very opposite of Mesiras Nefesh! The words Messiras Nefesh mean ”Giving over of the soul” and this does not require death or pain to be real. It can be achieved even during the highest levels of Simcha (joy). It is simply the desire deep in the Jewish soul to give everything away to G-d, to let completely go of the self, as Dovid Hamelech said “to you G-d, I lift up my soul”.
Of course, when a Jew dies al Kiddush Hashem and cries Shema Yisrael, he is also achieving true messiras nefesh. The real meaning in the words “Hashem Echad” are that there are no other desires or “self” other than G-d. The words “Hashem Echad” don’t just mean that there is only one G-d. Every 5 year old can tell you that. “Hashem Echad” really means that we want to give ourselves completely over to G-d so that our will no longer exists as separate from G- d, but rather that there only exists G-d’s will. When a Jew gives up his very life with this intention, he is giving up his entire self to G-d and causing the greatest “unification” of G-d possible. “Hashem Echad” really means to say that all I have, all I do, and all I desire – it should be G-d’s will. I am His; there is only Him. That is why Kriyas Shema is called “messiras nefesh” in the holy books.
When we read Kriyas Shema twice a day, we can have lofty thoughts of messiras nefesh while either imagining that we are being thrown into a fire for G-d’s sake, or while we are rejoicing in being children of such a great and awesome G-d. It doesn’t matter, for when a person truly gives themselves over to G-d in his heart, they wouldn’t care anymore whether they are given everything by G-d (even all the pleasures of the world!) or whether they are being thrown into a fire. Because when it’s not about “me” anymore, there is only G-d's will.
Of course this is a very high level to reach, but perhaps this is similar to what we can reach on Purim through drinking and rejoicing until the point where one no longer knows the difference between "cursed is Haman and blessed is Mordechai". What do our Sages mean to say with this example? When a Jew is at the level of messiras nefesh – which is made easier on Purim through the high levels of joy we reach (and with the alcohol also helping us forget the “self”) then there no longer exists any evil. For no matter what happens, all is G-d.