Do you remember your Math classes at school when multiplying two negative numbers you get positive answer. It is the same here. I have googled about more information and I have found that that is called Negative Polarity (the grammatical character of a word or phrase, such as ever or any, that may normally only be used in a semantically or syntactically negative or interrogative context ).
The full phrase is cannot help but do something and it is formal. You can make it less formal omitting help or using can't instead of cannot.
Similar idiom is I am forced to something, though .
Example: I am forced to the conclusion that Fred is guilty, though I'd rather not believe it .