Why my girlfriend won’t listen to me when I told her that she can't have another tattoo?

She already have a tattoo when is 18 and she never told me.

Do you own her?

If you don’t, there is the reason.

She does what she wants.

She can have another tattoo if she wants one.

You think just because you're her boyfriend you can tell her what she can and can't do, I hope she gets that tattoo and while she's at it I hope she gets a new boyfriend!!

You don’t own her.

You telling her that she can't have another tattoo might have given her more reason to get one.

I personally don't care for tattoos, but your girlfriend is free to get as many as she likes .... And you are free to accept it or walk away. 

I don't care for tattoos.  It's been proven that they are addicting.

Of course you can't control her but perhaps you can make suggestions about a smaller one and less readily visible one on a more discrete area of the body.

Lol... You ' ll have something to read and look at in the morning in bed instead of the newspaper cartoons.

Apparently she has forgotten about that power of attorney she signed giving  you control over her life.

Have her read it again.