I was hoping someone would find what I haven't been able to find: namely a duplicate of this question. But the only questions/answers I can find are related to various particular scenarios.
There are various constructions similar to ...しか...ない
in Japanese. Some call these negative polarity items. The linked answer gives some other examples in both English and Japanese that may help you wrap you mind around the concept. And as @aguijonazo mentioned in the comments, this particular construction can at times be reworded into a corresponding negative construction in English.
But, here I'd just like to mention the following: There are various ways of saying only in Japanese. The ...しか...ない construct is one of them.
I eat only apples.
I only read Japanese novels.
I could only get there by train.
しか is applied to the noun phrase (after a particle if there is one) that is being restricted. In English, the translations can be a bit more lose.
Other ways of expressing only in Japanese can be formed using だけ and ばかり.