Selphie meets Seifer when she is assigned the mission of delivering a message to him during the SeeD field exam in Dollet. Because his squad wasn't securing the Central Square as it was supposed to, and also due to his weird actions during the whole scene, Selphie starts off finding him rather strange: why would he be so careless as to ignore the orders to retreat and keep running? These factors, and the fact that she never gets to actually establish a conversation with him at the time, make it impossible for her to form an opinion on him right away.
However, things change: in her diary, despite admitting to having found him to have gone "berserk" during the mission, she also mentions how he congratulated the new SeeDs after the results were out, despite not having passed the exam himself (am I really the only one thinking he was just being his good old sarcastic self? Seriously, am I?); assuming him to be a nice guy, she wishes him luck on the next exam, going to the extreme of having very hilarious delusions concerning his potential participation in the organization of the Garden Festival.
Naturally, we must keep in mind that, having just transferred from Trabia Garden, Selphie is unaware of Seifer's background, attitude and reputation, and thus also of the fact that there is not much to like about him; still, it does not take her long to open her eyes and realize the view isn't too appealing: this is an arrogant, selfish person who would sacrifice it all, including innocent people and things he should have no real control over, for a cheesy childhood dream. Seifer is a disturbed, fragile young man, and this makes him an easy victim of manipulation: throughout the game, convinced that he is earning Ultimecia's love or respect, he commits all sorts of horrible deeds with no actual visible redemption (no, fishing doesn't really count as redemption in my book...).
I am certainly not the moralist kind: all humans are bound to fail and Seifer is young enough to start his life over; however, this analysis is based on his actions during the course of Final Fantasy VIII, and as far as those go, things don't look too shiny. Selphie is well aware of this, but it seems to me that she doesn't hate him half as much as she pities him: which is only natural. Before the second battle against him near the end of Disc 2, when he attempts to mess with Squall's party's minds, Seifer tells Selphie it was too bad that they never got to know each other better
(worst pick-up line ever, really bad timing). Her sarcastic, yet firm answer makes her feelings towards him very clear: "Nahh, you're not my type".
Sadly for him, Squall & Co. find Seifer to be more of an annoyance than anything else, failing to give him the attention he eternally yearns for: even when he's in the spotlight. Because they realize he is, in fact, just plain weak, they would probably be able to forgive him later, were they given good reason to. Selphie would likely find this easier to do than most others, not only because it's part of her personality but also (and mainly) due to the fact that she hasn't had to put up with him for the whole of her teenage life.