Why does Shivji sit behind his own Shivalinga?

Shiva worships the Shivalinga

This depiction is a favourite Shaivic imagery. it is extremely beautiful and full of spiritual meaning.
Why does Shivji sit behind his own Shivalinga?

Because,in the simplest words: He is committed to His own creation. He is His own first votary.
If you compare this to a modern day analogy, where during an election, the contender for head of state also goes to caste his vote, and ,of course, he's up early, one of the first to caste his selection! leading by example..he believes in His own manifesto, and stands by His principles.
But this example i give only to make you understand that a Good leader believes in what he stands for, he signs up for the teachings and Vision he gives us.

But of course, the analogy ends here, you cannot be "voted" God !
And although i truly loved reading them, I do not personally subscribe to some stories theorising that Shiva was a man , raised to the level of God by his followers. Rather, my work is based on the understanding that Shiva was always God, but endearingly Human, lovable like a good man in our midst! My understanding is based on the notings of the Ancient Rishis:
They said a surprising thing in the Vedanta and Upanishads: You cannot Become God, You are always that!
What did they mean by this?
Their teaching was expanded for us by the Rishis of Kashmir in their Shaiv Darshan.They explained that Sometimes, God makes Himself Forget that He is God, just before he descends to Earth. This paves the way for play- Lila.
So God looks for God! and finds Himself out!
According to them, God finds His own true nature when He is in deep Dhyana, Samadhi and it is that great realisation which the great Adi shankara termed as Chidanand roopam shivoham. It is also called Satchitananda or Satyam Shivam Sundaram.

We note also, that ultimately we are all seekers of the same truth and helped by Gurus and saints who have journeyed before us.
When He comes on the same journey, In this lovely Lila, God is also helped by His own Rishis as Gurus!
The teacher becomes the taught! This is not difficult to understand, Rather, this is the real beauty of God's Lila and love.
That is why you have examples of Ramji being taught by Vashisht, Krishna being taught by Sandeepan: so that they may remain centered in their Godhood.

Coming back to Shiva and the Shivlinga, Therefore we see that Shivji respects what his own Shaivism stands for, He is the first adherent.He leads by example and from the front.
Aum Shiv Shakti Namah.