ohh this is really interesting. i hadn't ever heard i was human before, so i went and listened to both that and shoujo rei before i started answering this.

reminds me of how back in the day when producers weren't as well recognized as creators (this obviously being a bad thing) people would assume songs were related when they weren't. a pretty good example was nem's dream-eating monochrome baku and fummy's why can't i even dream. they both have to do with dreaming, obviously, with the faceless girl (assumed to be miku) at the end of dream-eating monochrome baku being left without the ability to dream after len eats all of her dreams -- both her nightmares and her good dreams -- and then leaving her to see the world without 'color'; color here being an analogy for dreams, since you can't have someone take away your nightmares and choose to live in a world of only good things. miku, during why can't i even dream asks, "where are you, my prince?" as len, who tricked her into falling into love with him, has taken what he needed from her and deserted her. it makes sense, right?

these songs obviously aren't actually related, it's a coincidence resulting partially from the fact that no one cared to really pay any attention to the names of either of the producers. dream-eating monochrome baku has an actual sequel, by the way, nemurihime, in which it's revealed that len is collecting dreams from humans to save gumi, a sleeping princess he's actually in love with, hence the title. but still! what about miku?

if you go back to the root of both of these songs, dream-eating monochrome baku and why can't i even dream (which i will be calling yumekui and yumemite respectively from now on as that is a lot easier to type) discuss this topic of what it means to dream in the face of loss. at the end of yumekui, len sings, "why the painful face? this is reality!" while miku in yumemite laments her inability to look away from what is real. she can't even dream of this person she's lost anymore because their loss is too real. len's song here is more of a cautionary tale on what happens if one spends too much time dreaming and ignoring bad things instead of facing reality. i guess what i'm trying to say here is that both touch on this concept of dreaming as escaping from a harsh reality: trying to do so and failing, the overindulgence in doing so to the point of becoming completely out of touch. i'm sure both fummy and nem had thoughts about this topic while writing both of these songs, and that's why people assumed yumekui and yumemite were related.

(a note: fummy also created the witch's house which seems to have to do with the central themes and message of yumemite, but i haven't ever played it so i wouldn't know. just a fun fact, i guess.)

now about the two songs you specifically mentioned: i was human and shoujo rei.

flowers are a pretty universal and commonly used symbol for time, and with it, death, as these two concepts are essentially inseparable (as the passage of time implies decay and death the same way it implies growth and life). the speaker in shoujo rei compares her "friend" to a flower because of the way she delicately lives, grows, dies, and decays, right in front of her. like most of the lyrics of shoujo rei, it's a kind of cruel comparison under the guise of being a gracious one. and the lyrics of i was human very blatantly connect flowers to death: "the flowers, the flowers, the flowers are falling / my beautiful days have died just the same." from these lyrics we can assume the falling flowers are dead (which i know is probably obvious, but when you write analysis and theory, clearly stated reasoning is important) and when someone's days 'die', i can't think of any other possible explain other than said someone having died too. reminds of me of this lyric in shoujo rei: "a flower vase placed on the next target, and the one who started it... was me."

forgive me if you already knew this, but in japanese schools flowers are put in a vase and placed on a student's desk in the event of their death, and it used to be a form of bullying to put flowers in a vase on a living student's desk as if to say, "i wish you were dead." (this doesn't really happen as much anymore, as far as i can tell.) this symbol pops up in a lot of different japanese media. from this i can only assume the speaker in shoujo rei has placed a flower vase on the desk of her "friend" anonymously, the beginning of her driving this girl to her eventual suicide while pretending to be her friend. (vocaloid lyrics wiki has a note about this which i didn't even realize til right now.)

but you didn't ask for analysis, and i'm sure the nem/fummy example seems pointless right now.

what i'm trying to say ultimately is that vocaloid music is uniquely philosophical in a way that a lot of other music isn't. vocaloid producers write music without any kind of filter. it follows that a lot of vocaloid music is raw, honest, and vulnerable and comes from very personal places and lines of thinking. (i am reminded of kenshi yonezu during this interview which i highly recommend reading if you haven't, i reread it often.) it makes sense that somehow, in response to their surroundings, vocaloid producers end up following similar lines of thought and writing music about similar things, with similar symbols and words and images. it's not out of the question that these producers were inspired by each other, either. this maybe sounds dismissive ("we're all human and we're all going to end up thinking the same thoughts somehow") but i think of it more as being very sacred common ground shared between people.

shoujorei and i was human both exist in the aftermath of death. in shoujorei the death is obvious, the suicide of the "friend", in i was human, it seems to be more of a spiritual death or perhaps a physical death that has left the speaker a ghost (whether this is metaphorical or literal is up to you), just like the one in shoujo rei. death is unquestionably the most common of all human "experiences" (philosophically speaking, death is not something one experiences, because you can't experience something and be dead at the same time, but i digress). feeling lost and alone because of the actions of someone you once trusted is another quite common experience, and i'm sure that someone who has experienced that might think "what was going through that person's mind when they manipulated and betrayed me like that?", and so they might write a song about either one of these things. the end result is a sort of never-ending, spiraling discussion that characterizes vocaloid music as a whole. two songs by two different people connected simply because both people happen to be alive. i think it's very beautiful.