When people get defensive over their choices that you inadvertently cast doubt upon, and they don’t want to admit (to themselves or anyone else) that they made the wrong decisions, they will often attempt to convince themselves (and possibly everyone else) that your opinions are invalid by discrediting you.
Hence, fanboy: a derogatory term that means someone who is blindly and irrationally devoted to a product that I believe is inferior to what I bought when faced with a similar choice, and whose opinions and arguments can therefore be completely disregarded.
I’ve always believed that the iPhone is the best smartphone out there. But honestly, what I’m really saying is that it’s better for me. There are some things that Android has which some users like and look for, which they don’t find in the iPhone. But for me, the iPhone is everything I want in a phone.