It's often difficult to think about humans' utility functions, because we're used to taking them as an input. Instead, I like to imagine that I'm designing an AI, and think about what its utility function should look like. For simplicity, let's assume I'm building a paperclip-maximizing AI: I'm going to build the AI's utility function in a way that lets it efficiently maximize paperclips.

This AI is self-modifying, meaning it can rewrite its own utility function. So, for example, it might rewrite its utility function to include a term for keeping its promises, if it determined that this would enhance its ability to maximize paperclips.

This AI has the ability to rewrite itself to "while(true) { happy(); }". It evaluates this action in terms of its current utility function: "If I wirehead myself, how many paperclips will I produce?" vs "If I don't wirehead myself, how many paperclips will I produce?" It sees that not wireheading is the better choice.

If, for some reason, I've written the AI to evaluate decisions based on its future utility function, then it immediately wireheads itself. In that case, arguably, I have not written an AI at all; I've simply written a very large amount of source code that compiles to "while(true) { happy(); }".

I would argue that any humans that had this bug in their utility function have (mostly) failed to reproduce, which is why most existing humans are opposed to wireheading.