Musar su Salmi 100:2

JOY: The second [branch of love of G-d] is joy, it is a great, essential principal in serving G-d. This is what David exhorted us saying: "Serve G-d with joy, come before Him with song" (Tehilim 100:2), and "the righteous will rejoice, they will exult before God and delight with joy" (Tehilim 68:4). And our sages said: "the Divine presence rests on a person only through his rejoicing in a mitzva" (Shabbat 30b). On the aforementioned verse: "Serve G-d with joy", our sages said in a Midrash (Shocher Tov, Tehilim 100): "Rabbi Abahu says: 'when you stand to pray, your heart should rejoice, for you are praying to the Almighty of whom there is none like Him'".

There are two diametrically opposed verses, each one of which purports to tell us how to serve the Lord by prayer. In Psalms 100,2 we are told: עבדו את ה' בשמחה, "Serve the Lord in joy," whereas in Psalms 2,11 we are told: עבדו את ה' ביראה, "Serve the Lord in trepidation, reverence!" The plain meaning of the verses in question is that they apply to prayer which is called עבודה שבלב, "service with the heart." Our sages have said that while praying, we must direct our heart toward Heaven and our eyes towards Earth: Our downcast eyes to remind us of our puniness, our insignificance before G–d the Exalted. Our uplifted heart will induce fear and trepidation so that we may serve the Lord out of true reverence. Once we have attained this level of reverence our heart will contemplate what is above us and thus will be full of joy that we have been found fit to serve such a Master. We note that both emotions are an integral part of prayer.

Abraham was very worried that Isaac might not accept death entirely willingly due to the physical pain involved. In that event he would turn out to be a blemished offering instead of an עולה תמימה, a perfect offering. This is what his prayer was all about. He was not concerned with G–d revoking His instructions, but he asked G–d's help to make the death the experience it was meant to be. Abraham simply invoked the principle הבא לטהר מסיין לו, that he who sets out to purify is entitled to a heavenly assist (Yuma 38b et al). The entire prayer was that both he and Isaac should be able to fulfil the imperative עבדו את ה' בשמחה, "Serve the Lord joyfully," as we have been taught in Psalms 100,2.