Important verse of Srimad Bhagvadgita-10

O best of the Kuru dynasty, without sacrifice one can never live happily on this planet or in this life: what then of the next?......4:31

हे कुरुश्रेष्ठ अर्जुन ! यज्ञ से बचे हुए अमृतका अनुभव करनेवाले योगीजन सनातन पारब्रह्म परमात्मा को प्राप्त होते है । और यज्ञ न करने वाले पुरुष के लिए तो यह मनुष्यलोक  भी सुख दायक नहीं है,फिर परलोक कैसे सुखदायक  हो सकता है?......4:31

That person who does not perform yagna or offerings of worship to propitiate the Supreme Lord and who does not perform the regular and occasional duties as prescribed in the Vedic scriptures  or  station in life. Such a person derives absolutely no benefit for receiving a human existence in this world. They have wasted a very precious opportunity as a human birth is very difficult to receive and humans are the only species on the Earth with freewill and the power to reflect on the Supreme. All the goals which humanity aspires for will also allude them for without following the ordinances of the Vedic scriptures which bequeath promotion in the material worlds how can one who never performs yagna even on the level of the food they eat ever expect to be promoted to the heavenly spheres or ever become qualified for the ultimate achievement of moksa or liberation from the cycle of birth and death in the material existence. Moksa (liberation from Bondages) is the true goal of all human endeavor and real purpose of human existence. 

Whatever form of material existence one is in, one is invariably ignorant of his real situation. In other words, existence in the material world is due to the multiple reactions to our sinful lives. Ignorance is the cause of sinful life, and sinful life is the cause of one's dragging on in material existence. The human form of life is the only loophole by which one may get out of this entanglement. The Vedas, therefore, give us a chance for escape by pointing out the paths of religion, economic comfort, regulated sense gratification and, at last, the means to get out of the miserable condition entirely. The path of religion, or the different kinds of sacrifice recommended above, automatically solves our economic problems.. When the body is fully supplied, naturally the next stage is to satisfy the senses. The Vedas prescribe, therefore, sacred marriage for regulated sense gratification. Thereby one is gradually elevated to the platform of release from material bondage, and the highest perfection of liberated life is to associate with the Supreme Lord. Perfection is achieved by performance of yajna (sacrifice). Now, if a person is not inclined to perform yajna according to the Vedas, how can he expect a happy life? There are different grades of material comforts in different heavenly planets, and in all cases there is immense happiness for persons engaged in different kinds of yajna. But the highest kind of happiness that a man can achieve is to be promoted to the spiritual planets .

By learning the yagyam(sacrifice of sensual gratification) by a competent guru one can know the reality of yagyam, by just wasting tons of milk or by burning the fire wood would not be qualified for yagyam , the real meaning of yagyam is karma doing the Rightful action that is cleansing of subtle mind to purity. Here lord krishna says that what ever is remanant of the food from yagam one should eat is equal to amrit or nector. भगवान् ने  उपयुक्त यज्ञ के रूपक में परमात्मा की प्रप्तिके ज्ञान ,संयम ,तप ,योग ,स्वाध्य ,प्राणायाम आदि ऐसे  साधनों का भी वर्णन किया है जिसमे अन्नका सम्बंद नहीं है । इसलिए यहाँ उपयुक्त  साधनों का अनुष्ठान करने से साधको का अन्थाह्करण शुद्ध  होकर उसमे जो प्रसाद रूप  प्रसन्नता की उपलब्धि होती है , वाही यज्ञ से बचा हुआ अमृत है , कियों की वह अमृत स्वरुप परमाथ की पार्थी हेतु है और वह भाव से उत्पन्न सुख में हर समय तृप्त  रहना ही यहाँ अमृत का अनुभव करना माना जा सकता है। And this is the secret of happiness. also refer 2:64-65,18:36-37..