Ok apart from Seiji being a total D-bag and the plot being a complete mess, there's something else that really annoys me in manga like these.
I became really fond of the bond developing between seiji and shou in the first 'arc', but ofcourse there had to be that one incident where one can predict the outcome of the entire manga series.... the first 'fated meeting'. I suppose it has sort of become a standard manga rule of sorts, where the protagonist (seiji) is fated to (in the end) fall in love with the first girl that they meet in the manga. For instance, the first girl that engaged in conversation with seiji was Yuki. And after roughly 150 chapters of 'fillers' nearing arguably (and tbh hopefully) the last arc of the series, Seiji ofcourse falls in love with Yuki. I don't mind it happening at all even though I prefer shou, but it gets annoying when it happens in EVER SINGLE romance manga.
Nisekoi: Regardless of it's harem setting, chitoge, the first girl that ichijou meets gets the most screentime and romance plot. And it seems that it will eventually turn into a relationship between them, unless the harem element takes over.
Kimi no Iru machi: In the first chapter, Haruto meets yuzuki and after going through 2 other relationships he ends up with yuzuki, much like what happened in GE...
it probably applies to other series as well, but when you start to notice that it's becoming a basic element in every series,you can't enjoy it anymore....