She had to stay strong when every girl they passed on the street stared at Jason in awe and amazement while glaring at her with hate and jealousy. Reyna claimed she had moved on, but Piper wasn't so sure. She recalled how she'd felt about it when they were on their quest aboard the Argo II: They were so easy together, it made Piper's heart ache. The point was, the way they fit together was just so...perfect. They trusted each other so much now, and Annabeth would be crestfallen, so heartbroken, if she knew Piper felt this way. But let's not forget about Jason: think about how he would feel if he knew Piper's feelings had shifted from him to Percy.
Piper didn't want to think about how much more emotionally screwed up she'd be if she let her emotions take control of her. she asked silently as she made her way to the arena.
So, obviously finding a huge brain, a fantastic face, a funny bone, rock-hard abs and a six-pack all wrapped up in one package of a boy is a little difficult. The idea is that the guy is literally something, or someone, from your dreams, and is therefore utterly unattainable and nonexistent.
So, naturally, pining after someone who doesn't exist your entire life is fairly unhealthy, and could ruin the chances of many girls if they kept up those foolish thoughts. So most girls probably stop thinking about, or at least completely obsessing over, the idea of a perfect dream guy before they get too old.
A dream guy, for those who don't understand, being the perfect boyfriend that a girl imagines.
The guy could be regarded for his brains, his looks, his body, his humor, or something else, but most likely it's a combination of all of those.