I think Tom Peck in the Independent captured the problem quite well, even though his is essentially a humorous column:

====

What better way than for the new Brexit secretary to show how things would be more grown up now than to set up his first day of negotiations in Brussels with an opinion piece in the Daily Mail which he described as his "Brexit war cry".

When the sole purpose of your visit is to sell Theresa May’s pre-collapsed Chequers "agreement" to the EU, the one that involves convincing the EU we can definitely be trusted to collect their import taxes for them even as we simultaneously threaten not to give them the £39bn we’ve already agreed to pay, why not kick things off with a nice war cry?

====

Leaving the levity out of it, persuading someone you can be trusted while backtracking on an earlier - provisional - agreement is a hard sell.