It's one thing to understand
why it is forbidden for a cart to be pulled by a little old donkey
and a big powerful ox at the same time. It's also understandable
why Hashem would look down upon grafting two trees together
or breeding two different animals together. Hashem wants
to keep His creation pure. But why is it forbidden to
wear a garment that mixes wool and linen?
The Mitzvah of Shatnez is another one of those Chukim: that is, a Mitzvah that comes with blind faith and no official reason given. Luckily, the sages have
whipped up a good few reasons for Shatnez anyway.
It all goes back to the first sons, Kayin and Hevel. On the 14th
of Nissan, Year One, Adam huddled with his two boys. He explained
to them that in the distant future, the Jewish people would offer
sacrifices on this very day (Pesach
/ Passover). Kayin came up with the idea that they too should
offer korbanot. Hevel agreed.
Kayin was a farmer. So he brought flax seeds for his korban. Unfortunately, he made the mistake of bringing the worst cut of his crop. Hevel, on the other hand,
brought the finest sheep from his herds. As they stood in anticipation, Hashem sent a bolt of lightning (actually a heavenly fire) to burn Hevel's sacrifice, but Kayin's sacrifice was not accepted.
The rejection of Kayin's offering burned him up. Kayin got so jealous that he
murdered his brother. Now, here's where the Shatnez connection comes in: Kayin offered flax seed; Hevel offered sheep. Flax seed makes linen; sheep grow wool.
Hashem declared that never again would the opposing spirits of Kayin and Hevel mix. Not even in clothing. Therefore, we are forbidden to mix linen and wool in clothing or even on couch covers.
On another level, Kayin and Hevel represent the forces of
tum'a (impurity) and kedusha (purity). When
their offerings are mixed together, it creates a spirit of tum'a.
The commandment of Shatnez prevents us from bringing
more tum'a into the world. The only garments for which
you are permitted to combine wool and linen are tzitzit
and the kohain's garments - the avnait, choshen
and aifod (belt, breastplate and apron). That's because
these garments are spiritually perfect, created on Hashem's
instructions purely for the performance of a Mitzva.