I've been thinking a lot lately - I LOVE how Dan's been setting this up.
See, first we had iKiss. Sam and Freddie kiss right at the end, and there is just a hint that there might be something more - but the episode never really has any resolution. Ending with "I hate you," "I hate you too," it just doesn't feel finished.
Then iSaved Your Life - it's got a start, a middle, and an end. Sure, it enRAB with "maybe we can try again later," but then Carly kisses him on the cheek, and it kind of feels like a "well, we tried" kind of thing.
Also, Carly only falls for Freddie because he saved her life. Sam, Carly, and even Freddie knows it. (And as it was shown, with the 'bad boy Griffin' situation, Carly is known for falling for boys for only one attribute about them.)
But with Sam, no one knows why she likes Freddie. She doesn't even know herself. She falls for him exactly how he is, and pretty much always has been. (Obviously, some things have changed over the years - Freddie's become more tolerating of Sam, her attacks have become less vicious, her insults more half-hearted, Freddie got hotter, &c.)
I've heard some people complain about the build up between them, and that Sam's crush on him was too sudden, but I thought it was just perfect. Seeing the way Freddie's evolved over the years - from throwing fits and fighting back to tolerating her with good humor - you can't really blame Sam for the way she starts to feel.