Well, trying to sum it up..
Amu's love for Tadase were a puppy love, the kind where you try to look your best.
Amu weren't in love with Ikuto from the start, there for, she could yell at him, she could be her honest true self, not acting all sweet. There for her love for him could be deeper, because she felt at ease not having to pretend to be some one..
While Tadase after turning her down, started to be interested when her feelings for Ikuto started to develop. But Tadase were mainly interested in one of the persons she could be,(with Ran) the happy go lucky girl, he admired the kind of person she were with that, so it weren't really love at then, but he slowly starts to feel 'love' for the real Amu, which also Ikuto had started to, there for telling her to grow up faster, so that she would bee in a more suitable age and understand his feelings towards her.
I'm following the manga, which came first, and didn't remove or add characters and stuff..'' (just so you know)
Amu had many moments with Ikuto, I don't remember if they all where there in the anime but like..
link
where he tells her he likes her, but since she believes he's just joking she doesn't take him serious, like when Tadase did it.
And there's more scenes like when Ikuto leaves and kisses her, telling her something like,
"I'll definitely make you fall in love with me, so prepare yourself"
Which were a bold move to make, but effective.
link <- picture
but well that's a lot of 'moments' but they're aren't the one showing the truth..
it's Tadase himself, when he's in his older version,saying he haven't seen Amu in a long time...
Because Tadase let's Ikuto and Amu be a pair instead of fighting with them about it, since he loves both Amu and Ikuto (as a brother) and realizes that Amu already loves some one else now..
Thou Amu being the easily embarrassed girl she is, blushes a lot no mater if she's interested in a guy or if he just talks to her.. hehe
when the teacher gets married, it seems like there is a tie, but actually Ikuto already won Amu over, or more exactly.. Amu were the one winning Ikuto over.. =w='
but he's the one she cries for, the one she's hiding even if the others doesn't like him, because she cares about him.. and do just more than care.. TwT'
And Utau stops chasing Ikuto too, after she and Kukai get's together, and.. Nageshi..ko...? Ugh... what now what's his name x'D ended up with Rima (very cute together I think)
and the samurai guy and Ya..Yu..Yaya? the baby girl? x'D
But well, that's how the story goes, but ofcourse people can look the other way when they like other pairs.. but that's how the manga ended :'/