Ever since he and Twilight Sparkle had first moved to Ponyville, Spike's had a crush on Rarity. At first he'd thought it was a secret - the big secret in his life - but he'd soon come to realize that the signs he had been displaying since day one were...really, really obvious. His first reaction had been mortification, followed by anger (after all, how could his friends have just let him act like that?!), but eventually he'd come to the conclusion that, sometimes, a friend within a circle just happens to develop a crush upon another friend, and that it's really no big deal.
But then something starts to nag at him: If Rarity's known about his feelings for some undetermined amount of time, how come she's spent all this time without even acknowledging that she does know? So, he decides to ask if he can come over for tea, with the intention of discussing these thoughts and feelings with her.
NOTE: Sex tag is for brief mentions of it during their conversation. There's no explicit talk, and it's not a recurring point of discussion.