unfortunately from the get-go MnH had to follow one of the basic laws of anime/manga:
the character with the most screen time is the one the main character will end up with. Therefore, even though there were "better" characters out there (Lucy and Ayase beat Midori hands down, in Love Hina Mutsumi was a better fit for Keitaro, Ranma had so many women WILLING to be with him that were more attractive than akane, yet ended up with her anyway, in Maburaho, Kazuki MUST end up with Yuna, even though the story begins to move towards the...other girl...it's too late in the series to do that)
about the only anime in existence that doesn't adhere to this law is Tenchi Muyo: across 6 different universes (OVA, TV1, TV2, 3 movies) it is never obvious which girl Tenchi will end up with. Though Happy Lesson ends up being sort of a Tenchi-clone in that sense
people will always root for the side characters, coming up with wacko convoluted proposed endings while a series is progressing (read some of the Konomini threads...) but when it ends, it's unfortunately usually very predictable. And the ones that aren't are either unsuccessful or, through progression of the story, had developed another character (if her NAME wasn't in the title i would have guessed Ayase could have "won" over Midori, hell i'd go out with her) to "take over" (I think Macross, if i'm remembering correctly, follows this line, in Macross 2 i was very surprised by the ending)
it also has to do with the process of writing, usually the author will map out the storyline ahead of time and will generally know the direction the story will take from the get go. Most writers in shounen romance manga are more comedy oriented and lack complex subtlety in their romantic interactions between characters. Which is probably one of the reasons H-games are so successful as anime/manga: the way the characters are written, anyone is fair game (though the anime conversions will, unfortunately, usually pick the "normal" or "best" endings.) Kanon had me on my toes while i was watching it for instance. Comic Party is the same way.
i've read volume 7 of Midori no Hibi, even though i couldn't understand half of what i was reading (my japanese is...not so good) i got the gist of it, and it seems really funny (kinda weird to be laughing at something you don't completely understand...)