"Chemistry: Either you have it or you don't."
Why thank you Dr. Meredith Grey, you truly are the Master of the Obvious.
Snide remarks against a popular medical drama aside, the first line is about as cheesy as it is true. When people interact, it is chemistry that invigorates the elusive dance of attraction, a invisible bondage of truly distinct people, solidifying a pack or collective to interact further.
How can one say that they have chemistry with their partner(s)? To put it simply, it's when everything just clicks or falls into place. Of course, as beings suffering from irrational biases brought about by unpredictable emotions, we tend to bypass the natural order of things. We entangle ourselves in a web of self-deception only to find out that we suffered a period of denial. On rare occasions, the feeling may be reciprocated, leading to a sinking feeling of mutual abandonment of the failed pursuit.
Of course, chemistry, in the author's most humble opinion, should never be limited to pursuances of romance. While the blue-eyed boy may not have chemistry with the brown-eyed girl, this does not necessitate that the two abandon all interaction. As it is, chemistry also applies to other types of relationships, positive, negative, platonic or romantic. To think otherwise is pure folly in the greatest sense and ultimately lead to numerous wasted opportunities and regret.