Your partner has always been in love with love itself. They have a well-developed fantasy about that moment when they find the person they've always been looking for, and their souls recognize each other. They might think they know just how they'll feel; they may even have dreamed of this moment and what it offers them.
The problem is, that fantasy could actually deceive them, if they find someone who fits everything they've been looking for, who pushes all the right buttons and makes them feel those ways that they've been waiting to feel. Why is this a problem? Because it might remain a fantasy, instead of becoming reality. Especially in the beginning of a relationship, we're all on our best behavior; we do or say just about anything -- not to deceive anyone intentionally, but to be the person we know our lover wants us to be. And the truth is, the standard your lover has set with their fantasy of the perfect love is a really high standard -- perhaps even an unattainable one. That's what could happen with this aspect between you, and those rose-colored glasses could slowly come off, revealing the harsh, disappointing truth. The person they thought they were involved with -- that perfect soul that was supposed to complement theirs so well -- turns out to be just a regular person, full of flaws. You both feel let down by the reality.