for a lot of reasons:
1.they REALLY have that kind of chemistry, so necessary for all couples...you can see that something is going on between them
2.i think their personalities fit very well, Amu's cool and spicy attitude and Ikuto's naughty and lonely side.their moments together have their kind of magic that makes your heart beat and say "Whoa, i wonder what will happen next?"They are both looking 4 a place in this world, and i hope that they realise that they are meant 2 be together!
3.it's destiny, just look at the lock and the key, and all of Tsukasa's affirmations...
4.they are soo cute together!(had 2 say that)
5.Ikuto is the one who TRULY loves her, not Tadase.You can see that in everything that he does for her!
There are many more reasons, but i'll stick with these...Amuto ftw, never forget that