The majority of the sedra ki tetze deals with things that should not be mixed/put together eg kilaim, shatnez, etc.
The Torah tells us not to wear shatnez (22;11)- this means wearing something which has a combination of wool and linen in.
What is the message of this law? The midrash says that this is based on the following…
When the 2 sons of Adam; Kayin and Hevel argued and ultimately brought the first murder to the world, it was because one brought wool as part of their sacrifice to HaShem, and the other brought linen.
Thus, as a reminder not to cause
machlokes, we are told never to mix these 2 materials – wool and linen; so we live with this message. We see here the devastating effects of machlokes, and the extent that the Torah goes to try and rid us of any vestige of meaningless machlokes and breaking down of interpersonal relationships.
After all, if HaShem is to be our Father, then we must agree to get on together as His sons.
(heard this on a tape from Rav Krohn)..