For those who are bewildered by Seddie, or just don't "get it," I will attempt to explain Seddie. I think the root is in Dan's interest in Sam's character. I think his ultimate goal has been to show that a girl/person of Sam's odd quirks and traits can be loved--even favored. However, he couldn't have thrown her at you and say: "Love her!" He had to start from scratch and build a progression using friends: Carly, and more in particular, Freddie. Freddie's perception of Sam was meant to mimic that of the audience, while Carly's perception of Sam was meant to display Sam's worth, even though we hadn't seen it yet--since Carly had known her 5 years prior. If you want to know what Carly sees in Sam, basically stay tuned and get to know her. If you noticed in iPilot, Carly says (to Sam) something to the effect of "now, why are you my best friend?" Our opinions of Sam can only evolve, mostly through Freddie, as Carly has always loved Sam, where as the integrity of her relationship with Freddie, and by proxy, her relationship with us--the audience--is in need of constant stress testing. And this is where Seddie comes in.
Carly is the example of a "normal" teenage girl--she's polite, she's slightly superficial when it comes to relationships with boys, and indulges in the activities of what some would consider girly. Sam, on the other hand, is brazen, crass, and tomboyish, and is "not like other girls." So of course, Freddie--since Dan can only speak through Freddie--was more inclined towards Carly, because we as an audience are more inclined towards normalcy. If [we] are going to love Sam, which I honestly believe Dan wants, then he can't force it on the audience, he has to build a progression, moving from what we're comfortable with (Freddie and Carly) to what we (initially) were not so comfortable with (Freddie and Sam.) If we were going to fall in love with Sam, then so must have Freddie. But there must be some foundation. I think romance was targeted as a means to make the progression more profound.
Anyone who has been in a close relationship (not necessarily romantic) knows that conflict can be a strengthening experience. We've seen this in Cam, and we've seen this in Sparly. Since Sam was a tougher sell to the audience, Dan created anchors using conflict between the characters. Note how most of the show's conflict has been conveyed through Cam and Seddie, where Sam is the common denominator in both relationships. Since Freddie is [our] proxy, more conflicts with him and Sam have been instigated. So by season 4, when we've gotten to know Sam and even like her, Dan could now introduce the premise of loving her character. Not only did Freddie date Sam, but he fell in love with her. If you've ever pondered the reason why Freddie wasn't as moral as Carly, despite them being similar in temperament, it's because Dan wanted to make Freddie flexibe and subject to Sam's influence, so that the iCarly audience could also be flexible and subject to Sam's influence.
Now in my saying this, I'm not inferring that the other characters received no development, but Sam was arguably the most developed character on iCarly. We know much more about Sam than we do about Carly, Freddie, Spencer, and Gibby. And Freddie, being the main male character, as well as Dan's voice for what he wants from the audience, works logistically in a romance with Sam. I'm not afraid to admit that Seddie's development is derived mostly from Sam's development as a character as well as [our] development as an audience. So for those who argue that Creddie is canon, it's the same as likening the audience to return to the comfort zone in which they resided, when initially watching the show, when Dan has spent 5 YEARS creating a new comfort zone. As long as Dan devotes himself to Sam Puckett, then Seddie should always make sense.
Who knows? Maybe I'm over-analyzing Dan's intentions...