Why only the feet?
Feet walk in the dust. They touch the earth. They step on the 'natural'.
Your head may think it, mouth may speak or eat it, hands may touch it, but ultimately, metaphorically, your feet bring you there.
Here is the picture - only the part of you that touches the worldly, natural, carnal gets soiled. Only the 'outer' can get soiled. Inside you, in your spirit, you are a new creation. HE SAYS you are a saint, holy and blameless, precious in His sight, absolutely righteous.
Consider this, the Biblical definition of repentance is to do a 180 degree turn from where you are to the exact opposite. We turn ON OUR HEELS and WALK the other way. It's a feet issue. If our feet didn't touch and interact with the natural we couldn't get dirty outside and believe lies about ourselves inside.
The key is, it is not about getting soiled, but rather that you ARE ALL clean. You are perfect in Him. He has perfected you, the real you, the spiritual you in union with Him - is complete. Only when we walk in the earthy, carnal, worldly do we get soiled. The cure? Washing off of that which does not truly stick, which can't change the truth about you...under the dust you are A MASTERPIECE.
Through years and centuries mankind has picked up grime that is lovingly, gently, perfectly and completely removed by a Master Restorer revealing once again the perfection below.
Was the soiled canvas any less a work of art or highly desired? No! It just required the Master to restore it.
We can't even cleanse ourselves. The painting can't fix itself and neither can we. We wait and trust our Master Restorer for He is faithful to do so.
Let HIM today wash your feet of the lies and behaviors causing shame and guilt.
Let Him close enough to cleanse each nook and cranny revealing to you, again, your PERFECTION and His great joy in you, His masterpiece.
As you let Him close, talk with Him. In intimacy and relationship with Him you CAN walk in the spirit and not satisfy the lust of the flesh. In the spirit your feet don't touch the ground.
Why 'should' we?
Not because it separates us from Him in 'sin' but it separates us from His truth and causes us to think we are something we are not - dirty and unacceptable.
The truest thing about you is what He says. He says, 'let ME wash your feet...let me prove to you that you are.....ALL holy, ALL perfect, ALL righteous'.