When people feel a 'spark', I am saying it's because they find each other physically attractive - that's, not a must, but it helps (as in, you can create the artifcial feeling of a spark even if someone doesn't find you that attractive, through behaviour, which, makes the whole thing a bit depressing and fake) and create a state of fluctuating anxiety in each other.

Physical attraction anxiety. Not knowing. Not knowing, quite, if the person 100% likes you or you 100% have them. Throwing innuendos, flirting, confessing in stages of your like to each other and not sure if it'll be reciprocated.

Much of the time, as humans, we feel the same physiological sensation in different situations, but just purely by the context and story we're telling ourselves... it can be good or bad. Think about it, 'butterflies' are physiologically, basically the same as anxiety - the anxiety you get if you have to give a presentation in front of a bunch of people. Except the presentation, you're telling yourself is negative and scary and you could look stupid, and don't want to do it... wheras 'falling in love', or 'spark', is fun, exciting and you want it.

This is why 'spark seems to die' after the honeymoon phase aka when people know for sure they like each other and that there's no mystery or wondering if the other person likes them, and they have each other. Then it becomes 'comfortable' aka no more anxiety/spark.

That's also why overly nice guys are not attractive to women, even if on paper they say they want that - because overly nice guys lay it all out from the beginning - they let the woman know - not only do they like them, but in a way that they desparately want them; which in itself is unattractive too - people want what they can't have, or have to seek approval for; if a guy is saying 'I WANT YOU, I'M YOURS, I REALLY WANT YOU' from day 1... there's nothing to be chased by, or chase yourself, nothing to wonder about, no fun, no mystery... no... 'spark', 'I just don't feel it... he's like my brother', that's why, haha.

Bit depressing when you realise all this,
Because makes you realise how fake it all is. Not fake like you're not feeling it, but sort of, trivial, the fact that this grand emotional whirlwind we obsess over and think is so deep and meaningful is just sort of... you can create it like a magic trick. Hence why hot and cold works, and anybody who thinks it doesn't is full of shit, I know for a fact it does bc I've done it, when I was young and learning all this... it works because it creates that state again of 'oh, hangon? they don't like me? I thought they did?' = anxiety. But anxiety in the context of romantic pursuit we call spark or falling lol... it's so... silly. Kind of sad. Humans are silly.