For the best part of their friendship, Mercutio wishes Romeo would just take one look at him; just once look in his direction and really notice him. It's not for lack of trying on his part, he thinks, but knowing that doesn't stop him from trying harder.
He succeeds in drawing Romeo into his orbit and keeping him around, though whether he has reasons beyond wanting a good time and a few MDMA pills, Mercutio can't say for sure. Benvolio has opinions on this, of course, but he's as much of a choirboy as you can be when you're a Montague, and he's deliberately intransigent to most of what Mercutio tries to say to him, so in the end he doesn't.
There's a part of him that has given up already, a part of him that has always known there is no point to this, and that he should just let it go. But the other, much larger part of him, is a caged animal that he's chained up for so long he's forgotten how to control it when it's loose. In between these two, he manages to convince himself that gatecrashing the Capulets' party is a good thing to do, a distraction, an opportunity to seek out a new adventure.
Except then Romeo shows up as a knight in shining armour, and Mercutio's heart constricts at the realisation that things sometimes just aren't that easy.
Romeo takes the pill from his finger. The red love heart on it melts on his tongue. Mercutio closes his eyes for a second because the image is too much, because Romeo's trust is somehow too much and too little all at once, because Romeo's under the influence but Mercutio isn't, he never is, much as he wishes for the plausible deniability that comes with it.
He realises well enough that he has lost Romeo the moment they walk through the door, but it doesn't really sink in until they leave without him at the end of the night.
"If wishes were horses," Benvolio says on the way to the car and Mercutio nearly elbows him in the face.
Some beggars just aren't meant to ride.