Why Do We Do Mitzva X,Y or Z?
The simple, straightforward answer is, and must be, that we do mitzvot because G-d commanded us to do them. On some level, that is not only the simple answer - it is the whole answer.

But we are human. And humans like/want/need reasons for what they do. And we are given reasons for many of the mitzvot. The better the reason, the more it makes sense to us, the more comfortable the reason is for us - the more we have to remember the simple, straightforward reason of the first paragraph. It is the mostreal of reasons for all mitzvot.

With 74 mitzvot in Ki Teitzei (the most of any sedra), we can find examples of different types of mitzvot and see how well or not reasons fit. Let's start with the prohibition of muzzling an ox when it is threshing. (We are taught that this mitzva prohibits muzzling any animal which is working with food.) Why not?

Because it would be cruel to the animal to prevent it from eating if it wants to. WRONG! You cannot give that reason before the "real" reason. First, G-d forbids it. That's why we don't do it. Then, we can try to understand why G-d gave us the mitzva. And in this case, our Sages almost did not hesitate to teach us aboutthe "kindness to animals" factor behind this prohibition (among others). For a mitzva such as this one, it was so easy to come up with that reason, so easy for us to accept and understand it, that we run a risk of keeping this mitzva BECAUSE of the kindness factor, and not because it is G-d command. And that's a bit problematic.

Don't wear garments made of wool and linen together. Shaatnez. A classic CHOK. A reason is hard to come by. There is a fair amount of illogic in the mitzva. Wool & linen, no but wool & cotton is okay? A sweater no, but curtains are okay. Our Sages came up with several "messages" that this mitzva conveys, but not "reasons"per se. It is a reminder of the clash between Kayin (whose offering to G-d was from the plant world, as is linen) and Hevel (whose offering was from the animal kingdom, as is wool). Nice reminder, but you really can't call it a reason. Why wool? Why linen? Some suggest an idolatrous connection to Shaatnez. Or l'havdil,a sacred aspect because of the garments of the Kohen Gadol. And as opposite as the last to ideas are, there might be a common denominator. Man rules over nature. But not without limits. Limits imposed by G-d. Specifically (but not exclusively), in the area of mixtures. There are plant-plant mixtures that are prohibited, animal-animal mixtures. And Shaatnez is the "token" plant-animal mixture that is prohibited. The chief fiber of the animal kingdom. The chief fiber of the plant kingdom. The chief human use of fibers - clothing. That's Shaatnez.

So how come we can weave wool and linen (as long as we don't wear garments of Shaatnez), but we cannot cook meat and milk mixtures? And why can we benefit from cross-bred animals (e.g. mule) but the grapes of a vineyard in which wheat grew are forbidden completely? We can have insights into these matters, but the bottomline is that we cannot know or understand everything.

Now let's look at one more example in this week's parsha. If you happen upon a bird's nest... Chase the mother away and take the eggs or fledglings... Act of kindness, you say. Let not the mother bird see you take her offspring. Maybe. But probably not. Kindness would be to leave the nest alone. Or if you say it is kind, then why only with kosher birds? Other acts of kindness apply to non-kosher animals too.

The Mishna (Brachot) tells us that if a person would pray to G-d for mercy, "as you show to the mother bird in Shilu'ach HaKen", we silence him. We cannot fathom the meaning of this strange mitzva (some actually suggest that it is a commanded act of cruelty, rather than of mercy!). This serves as a warning to be similarly cautious about assigning reasons to any mitzvot. Even the most seemingly logical mitzva can have other angles that are hidden from us. Mitzvot are G-d's commands. He is far too complex for us to understand fully.