Most of the time people relate it to chemistry when they fall (or fail) in love with others. I really don't know what kind of chemistry are they talking about. You know, I have my own chemical equation here. In chemistry they have a certain scale to measure the acidity or alkalinity of a solution, which is known as pH scale. Solutions at 25°C with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). When a pH level is 7.0, it is defined as 'neutral' at 25°C. So, let's imagine another scale here, and let's call it pN scale, where the letter "p" refers to politeness, while the letter N refers to how "Naughty" you are. And people here can be anything between 0 (So polite, or naive) up to 14 (Naughty). The point is, in order for a relation to succeed, both parties pN values, have to be as close as possible, with the man-party having a value slightly above that of the woman-party. If she is too naive to him, he will get really bored of her and she will get jealous all the time. Even in Quran, it's said the:
الْخَبِيثَاتُ لِلْخَبِيثِينَ وَالْخَبِيثُونَ لِلْخَبِيثَاتِ وَالطَّيِّبَاتُ لِلطَّيِّبِينَ وَالطَّيِّبُونَ لِلطَّيِّبَاتِ
Which means; "Good men are for those good women, and bad men are for those bad ones".
Ooops, what the hell did I say here? It's my non-technical blog, however I insisted in the first post here to talk about pH Scale, and Chemistry, and such holy crap.