Palpatine believes that, because
- He knows her values / what she stands for
- She has literally just said she doesn't want additional guards
- This appears to be an ongoing argument.
He knows her values:
The entire theme of the opening of Attack of the Clones is that Padme stands for freedom, ahead of security. She is opposing the creation of an army to protect the republic, because she feels that puts security ahead of freedom.
From the script / a deleted scene of her speech shortly after the assassination attempt:
I have led the
opposition to build an army... but
there is someone in this body who
will stop at nothing to assure it's
I warn you, if you vote to create
this army, war will follow. I
have experienced the misery of war
first-hand; I do not wish to do it
Wake up, Senators... you must wake
up! If you offer the separatists
violence, they can only show us
violence in return! Many will
lose their lives. All will lose
This is symbolically paralleled by the fact that she doesn't want guards for herself, since they would be a microcosm of surrendering her freedom for security.
She has literally just said she doesn't want additional guards:
PADMÉ: Chancellor, please! I don't want
any more guards!
I think this speaks for itself.
This appears to be an ongoing argument:
Chancellor, if I may comment, I
do not believe the...
..."situation is that serious."
No, but I do, Senator.
The fact that he's finishing her sentence indicates this is an old argument that they've had a few times, so he knows how she feels about having guards. Given her irritated response when he insists:
This is not necessary, Chancellor.
She is aware that he is aware that she doesn't want this.
Source: Attack of the Clones Script