"I didn't fall in love with it" is a rejection designed to make you unable to argue with it. There's something about it that made them not want to represent it, but they're not going to say because if they are fully honest, they're risking some writer shooting back an email with, "Well if you read to chapter 4 you'll see the settings are fully developed!" or "Mitzy becomes far more likeable if you just read all three books in the trilogy!" and "What do you mean, this plot has been done to death?" And the more subtle their reserve (ie, maybe they feel everything is great but it's just a shade too similar to something else they just sold, or they haven't been able to sell the last three projects with a French female attorney protagonist) the less likely they are to want to voice it.
So rather than tell you whatever subtle issue made them opt out, they tell you they didn't fall in love. That has the virtue of being true without the drawback of being specific.
Don't change anything about your manuscript because of that feedback, in other words.
DO laugh whenever you see some big publisher putting out a ridiculous book, though. Say to yourself, "I'm sure the agent and entire editorial board totally fell in love with this, and not with the money."