There's not much chemistry in Garden State, but maybe there doesn't need to be. If so many love stories are the right person at the (comically) wrong time, GS deals with not necessarily the wrong but not necessarily the right, possibly any (or at least any not clearly wrong) person at the right time. Sam doesn't cause a change in Largeman's life (though maybe she inspires him to keep it); she merely appears at the time it takes place, and if this is meant to convey fate, it more convincingly states that their meet-up (hook-up) is more about time than person. This is probably not the intent of director Braff; Natalie Portman's Samantha ("Sam," meant to mean 'quirky' name) is stocked with enough 'charming'-style idiosyncrasies that she's probably meant to signify uniqueness. Let us forgive her, though. If these quirks seem a little run-of-the-mill, they still signify a certain level of vitality, and a certain level of vitality combined with the primary factor - timing - is evidently plenty.
The film states early on (as early as the funeral) the familiar small-town (or hometown) trap, and puts Largeman back in the un/appealing haze of drugs/booze/high school girls; while it doesn't condemn it outright, it seems to be setting us up for the contrast of the sunnier world Sam will bring along. The portrayal is generous, though; it's easy to make the hometown party scene (the party scene and film scene both) completely unredeemable, but Braff lets it have a little appeal - the doting and at least somewhat genuine friends, the pretty girls, the allowance by Largeman of himself to trade his usual medication for more illicit kinds - and even if his smile is at least partly influenced by drugs and a large part by confusion, it's at least a small part drugs, booze, girls and home. Though he may be enjoying some part or parts of the experience, Braff presents it visually so as to remind us that even if it's a more pleasurable fog, it's still a fog, and maybe also proof that though this may be as far as he can get from his usual stumbly routine - ingesting medications designed to intensity feeling rather than nullify it - a fog is a fog.
The answer left to him is to seek the clarity somewhere between the antithetical fogs of feeling and unfeeling at either end of the spectrum. For this he needs to stop taking drugs. He does not need to meet a girl. If he should happen to do so, great, but his decision to leave his drugs behind is made before he meets Sam; he meets her only because he has already begun to enact the course of actions designed to free his consciousness from its pharmaceutical accommodations.
Is she a bonus prize, then, or is she the fated meaning? The latter comprises a high-demand series of assertions: that Largeman was destined to pull his act together and leave the drugs behind in the precise nick of time to return home where he would come across the girl destined to change his life in the brief window of time before he lost faith in non-drugged reality due to what would likely have been a series of strange and unfulfilling experiences with his father, his old friends, his hometown, returned to his fairly empty life and his fairly full medicine cabinet. This girl would be the only girl quirky enough to point out to him how weird and good his life might be. If the chemistry were a little better, that would be the assumed intent.
But there's not much chemistry in Garden State.
The result is that Sam does not seem the girl destined to be in that spot at that time when Largeman made that call. She seems, instead, like a girl. She's a nice girl. She's somewhat pretty and somewhat interesting and she doesn't have a boyfriend at the
moment; she doesn't seem to listen to Guns N Roses. Her lying, potentially a source of some deeper interest in the character and possibly a bond between the two - a legitimate mental condition - is played lightweight, and it's never clear whether their respective familial problems are meant to be a bond or merely the mark of all families and thus presented as not a reason for them to not get along.
There's nothing so wrong with that. As he comes out of his lithium stupor, he begins to recognize the value of small experiences, of anything not experienced through an antidepressant cloud. Like a smoker who has quit, the air seems cleaner; we do not experience this through dramatization of details (with a glaring, trite exception or two) but rather more retroactively, and when he explains it to another character late in the film, he is explaining it to the audience as well.
In any case, during this reawakening, a small experience comes along which benefits from this period of increased value: he meets a girl. She may or may not be anything special, but in his mental state, even her small amount of verve is enough to seem like a big deal, and he is willing enough to follow her enthusiasm that she becomes his tour guide through an alternate version of the Hometown Return, showing him that his brief tour through his hometown doesn't need to be what he (and we) thought it would be. Certainly she is presented as more appealing than the druggy girls at the party, and if he has begun to denounce that trap of old friends and old habits into which he might have fallen, Sam shows him that his journey through town need not be dictated by that default pathway. In a destiny-style love story, that would probably be the point, but it is still clear in Garden State that she is neither solely responsible for it nor completely effective, for when Largeman is reclaimed by his old friends for more parties and gatherings, she does not remove him from that scene but comes along instead. Just as druglessness adds to the value of her presence, she becomes a small amount of increased value to the party scene, her presence making the parties tolerable. She even seems to make his experience with his friends tolerable; it isn't until she comes along that Mark begins to seem redeemable, and though it's got nothing to do with her, it's once again due to framing. She didn't redeem Mark because she's not the catalyst for Largeman's life improving. Mark redeemed himself, or became viewable by Largeman as redeemed or redeemable, because Largeman's new viewing angle on the world allows space for him to find the redeemable aspects of Mark's flawed life.
If it had been Sam, if Sam was his destined true love (or whatever), she might have come along just as she did and just when she did. Their meeting does seem to indicate that, at least in fortuitous-meeting movie terminology. On its own, their meeting and a few of their interactions seem to hint at a traditional destiny-style love story, but the rest of the movie contradicts this, and when we finally realize that because of his mother Largeman does not believe in destiny, only dumb chance and accident, we know that Largeman and Sam weren't destined to meet, they just happened to, and maybe that's not so bad?