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I'm bad at English but I will try to convey my thoughts
Well I get what you mean but in the other hand when you think about it I could bet that those who gave 5 stars are a) new to mangas b) those who wanted something sweet and fluffy after some fucked up mangas marathon or c) just simple cliché lovers I mean those romantics.
When I was just getting into mangas (of course I started with yaoi and even now I'm hardcore fangirl) this was one of the first ones so I gave it 5 stars, though that it was amazing. Now I still think that the art is beautiful love the style but storyline isn't fascinating anymore its just meah not bad but not super good either. So if I had to rate it now it would not have 5 stars.
I suck at explaining sorry ( ̄へ ̄)
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I think that problem also lays with 1-5 stars, because I often feel that the manga was 4.5 but there is no such rate, you can give 4 or 5 (this feeling when you really like manga but it is not 5) and I then give 5 stars after all because I find that the author he devoted his time to draw it, to think up, and so other people or even translators, the rounding up of the rate does not do me any harm and will appreciate them.
I give this manga 5 and this was a long time ago and maybe I was more pure but I feel like even now I would end with giving this manga 5 :D but I still like it when I reread this, and I don't plan on changing this rate, because art is gorgeous, i like cliche stories without complications, simple whit a little sad moment, and most important for me I want to be honest with past myself who read this first time and feel this is worth 5 stars.
This doesn't deserve 8.9 rating in my opinion. So cliche. The only thing I like about this is the last part. That made me laugh.