"Cannot help but" is a phrase that is commonly used to express an involuntary reaction to something. However, if you want to add some nuance to your writing or speech, there are several synonyms you can use. One option is "unable to resist," which implies a more active effort to resist but ultimately failing. Another option is "inevitably," which suggests a sense of inevitability without necessarily implying any effort to resist. "Can't refrain from" is a similar phrase that focuses more on the action that is being resisted. Alternatively, "compelled to" suggests a sense of obligation or duty that drives the reaction. Overall, there are many different ways to express the same idea as "cannot help but," so it's always worth exploring different options to find the best fit for your context.