One should celebrate sacrifices from desire of heaven'-we always hear this Shruti. Thinking first of the fruit, one makes preparations for sacrifice.
स्यूमरश्मिरुवाच स्वर्गकामो यजेतेति सततं श्रूयते श्रुतिः। फलं प्रकल्प्य पूर्वं हि ततो यज्ञः प्रतायते॥