As we study and learn about Torah and our tradition many questions arise. This particular question, regarding the mixing of different breeds of animals, has been asked many times over the generations.
The Hebrew word kilayim means “of two kinds,” and is the term written in the Torah that is the source of the answer to your question. Anything “of two kinds” (mixing animals, seed or cloth) is forbidden. According to the rabbis the law forbidding the mixing of animals (or any other matter), found in Leviticus 19:19, falls into the category of chukim - a commandment whose purpose and meaning are not obvious or rational to us, but we must observe the commandment. (The 613 commandments are all commandments but can be classified into different categories.) One of the sages, Nachmanides, clearly states that the mixing of breeds is prohibited because no new species may be created; all species were fixed at the time of Creation. His view is supported by other commentators, including Rashi.
It should be noted that this particular prohibition in the Torah is found within the section known as the Holiness Code, Leviticus 17-26. The Holiness Code covers a lot of ground and includes laws for proper worship of God, sexual behavior, priestly responsibilities, festivals and holy days, and ownership of land. Overall these commandments teach us to aspire to holiness. As Rabbi Louis Finkelstein so beautifully states, “Judaism is a way of life that endeavors to transform virtually every human action into a means of communion with God.”
Thus, Jewish laws and rituals are a way to think about, and engage in, living holy lives, even if the approach or methodology does not always seem logical to us.