Probably over-simplified but ...

UK - we would like frictionless trade.
EU - fine but that means your regulatory system must harmonise with ours & if we amend our regs. you must amend yours. Also, if there is a dispute about the meaning of a regulation, only the ECJ can deal ..

.. with it because, otherwise, two parallel bodies of inconsistent case law will develop leading, in effect, to regulatory divergence.
UK - but we are a sovereign country. We can’t agree to that.
EU - then there cannot be frictionless trade.
UK - now see here, you sell us more ..

.. than we sell you so you’ll have to crumble in the end.
EU - not by undermining the SM & CU. Those are fundamental to our prosperity & part of the process of ever closer union.
UK - you’re acting in bad faith! You promised a FTA! You did, you weally weally did! You pwomised!

The ‘you sell us more than we sell you’ argument has a flaw. Can you see what it is children? Yes, that’s right. Interruption of trade means they don’t get our money, but it also means we don’t get the goods. And we need the goods pretty badly, as we will learn on 1st Jan.