JOEL: Random thoughts for Valentine's Day. Today is a holiday invented by greeting card companies... to make people feel like crap.
We're not disagreeing that Valentine's Day was created for commercial purposes, but it's a bit cynical to think it's purposely making people feel bad. We don't think there's anything wrong with a dinner for one followed by a Nicholas Sparks movie. But seriously, Joel's thoughts here pretty clearly reveal how he's feeling about love and relationships at this point in the movie.
JOEL: Sand is overrated. It's just tiny little rocks.
Okay, sand does seem to get everywhere after a trip to the beach, but isn't it still fun? Joel may be right, but that doesn't mean thinking of sand as merely a smaller form of something hard and boring like rocks isn't really depressing. What about sand castles? Or sand art? You can't make that out of pebbles.
JOEL: I could die right now, Clem. I'm just... happy. I've never felt that before.
JOEL: I'm just exactly... where I wanna be.
Joel and Clem are lying together on the ice when Joel experiences this moment of bliss. It's not complicated: he's just very happy, in a contented sort of way. This is the moment when he realizes that getting rid of all of the bad memories is not worth losing all the good ones. Will he ever get to experience happiness like this again?
JOEL: I love being bathed in the sink; such a feeling of security.
CLEMENTINE: I've never seen you happier, Baby-Joel.
Not all of Baby-Joel's memories are sad ones. He is really into sink baths, for example. If only we were all that small still… such a feeling of security. But we digress.
MARY: So you didn't do the erasing?
STAN: Of course not. God no.
MARY: And you never even suspected we were together?
STAN: Once, maybe. I was coming back from a job, and you were at his car. I saw you two talking. So I waved to you, and you giggled, you know.
MARY: How'd I look?
STAN: You looked happy. Happy with a secret.
Mary doesn't want to remember her affair with Howard only in order to avoid making the same mistake twice; she actually wants to keep the memories for the same reason Joel wants to keep his, specifically to remember her happiness (even if that happiness is ethically questionable).
CLEMENTINE: Too many guys think I'm a concept, or I complete them, or I'm gonna make them alive. But I'm just a f***ed-up girl who's looking for my own peace of mind. Don't assign me yours.
Clem is totally right. She knows that Joel thinks she will be able to make him happy by freeing him from his dull life. But even telling him straight up that they are in the same boat, both searching their own peace of mind, doesn't make Joel realize the truth.