Person  A is under the influence of sex pollen or aphrodisiacs or whatever the case may be. 

They've always secretly loved/had a crush on Person B, so they come onto them (by kissing them unexpectedly, calling them and telling them what happened, asking for help, etc)

 Unlike the general fluffy, smutty fanfiction however, Person B doesn't reciprocate Person A's feelings and they refuse to "help" A, leaving A alone feeling unwanted and desperate.  

we all have read one of those fanfics before