When you are a child friendship comes easily. You befriend others around you based on nothing other than the fact that they live nearby and happen to be in the same age group that you are. If you are lucky, they're also not annoying as fuck, but even if they are you befriend them anyway. You're easy to please. "I like dinosaurs," is widely accepted as a valid basis by which to establish a bond. You can pretty much replace dinosaurs with any other word you can imagine, such as dogs or rocks, and the argument would stand just as strong.
Then as we start growing up, usually around sixth grade or so, you start to notice things you had never noticed before. Your BFF likes blue rather than red. How had you never noticed this before? And it gets worse. You find out they like all sorts of things that you don't! Such as troll dolls. Or maybe they like Zack more than they like Slater. Suddenly your entire friendship is flipped upside down. Nothing makes sense anymore, but you hold on for dear life. You try liking those things too, it's your obligation as a good friend.
Eventually you just can't keep up. They go too far. You find out one day they like something you simply abhor the very existence of, like Hansen, and suddenly your BFF is hanging out with some other kid who loves Hansen too. You're left out.
Your friendship isn't over, however. No. Both of you for whatever forsaken reason refuse to let go. You don't hang out as much as you used to, and this results in the gap between what you like and what they like growing steadily wider. As your influence wanes the tension to somehow force your friendship to overcome these many tiny differences of opinion builds. Then there is a catalyst. Maybe it's that girl you find completely intolerable that your BFF has been hanging out with at lunch, or maybe it's a boy who appeared from no where and takes up all of her time. Whatever it is, it causes an EXPLOSION! The likes of which your friendship cannot survive.
You make a remark about this catalyst and your friend disagrees so completely that you can feel the resentment in their voice. It has become some tangible thing in the room with you. This is it. The true turning point. The point of no return. The situation elevates beyond control because neither of you are emotionally equipped to handle this sort of thing yet.
You tell your friend that you hate her. She says she hates you times infinity. It's over. At long last. While hurt, you are also somehow strangely relieved. At least you don't have to pretend to like her troll dolls anymore. You both move on to new circles of friends and lose touch. When people ask you down the line why you two aren't friends anymore, both of you will always say, "She changed." And that resentment will manifest itself again.
So what happened? Nothing. You did not change and neither did she. The two of you have always had different opinions, you just never felt the need to express them. You were too busy pretending to command spaceships and catch fairies in a butterfly net.
Think back to why you became friends in the first place. The answer is likely either, "She lived down the street," or, "We were in the same class." Your entire friendship was based on convenience. You shouldn't be so surprised things didn't work out. Of course your friendship fell through, you had nothing in common to begin with aside from age and perhaps a fondness for chocolate milk.
Also? You should probably get around to slaying that resentment monster. It was never warranted in the first place.
Disclaimer: This is written in the female perspective but easily applicable to both genders. If you have outdoor plumbing simply substitute all instances of she and her with he and his. Also, depending on orientation you may need to substitute Zack and Slater with Kelly and Jessie.