Yeah... They basically go by how they feel at the very moment. That's why the opposite also applies. They could not have any interest in you one day, and treat you like crap - then be all over you the next; completely obsessed and super nice. Completely out of no where. Or, you could be pining after them forever - only to be rejected - then suddenly find them loving you like no other. Or, they could not want to be around you, and find you boring - until you make them excited and horny. Then, their mood switches just like that. Or, you could be dating them, and making them super excited, but once they get bored even just once - they fall off the face of the earth - like you never mattered. The back and forth is constant, and there are several more examples for which this applies.
It's all about how they feel RIGHT NOW. They can pull a 180 very quickly, and turn right back again - depending on how they feel, or rather, how YOU make them feel. Don't invest too much of yourself too soon. You are always going to be the best thing ever for the first while. This can and does change at the drop of a hat. Don't make anything of it. Just do you, and keep the game where its supposed to be at all times. Don't stress or worry about this, either, which coincides with not attaching yourself too soon, or falling for theirs. Just enjoy yourself for as long as it lasts.
Never assume feelings are permanent. They do change, and you have no control over it. Don't think back to how things USED to be, because the only thing that matters is right now. Always focus on the here and now, and understand that it might not last forever. Ergo, don't get attached too soon. Once feelings change - that's a wrap. Do NOT let their emotions run the show, or allow you to determine what you should or shouldn't do. Don't bounce back and forth and try winning them over, or any of that stuff. Don't lose yourself to them. It's just going to make you crazy and confused - because they're gonna be all over the place. The only thing you need to be focusing on is their interest level RIGHT NOW. Nothing else matters. You can't change what is already gone. Never rest on your laurels, and assume its smooth sailing just because they like you for a while.