Simply because of their interactions and continual development throughout the series. Their relationship is never static and unproductive, meaning that both participants give and receive encouragement/affection. Another reason why I like it is because it shows a very understanding and happy couple.
Also because it is a relationship where neither one has to have double the load to get there, unlike the more commonly seen couples where the female has to always work the hardest to be loved while the male doesn't lift a finger.
I support EQUALITY in a relationship!
Of course there is alot more but I rather not get into details. PLUS the fanarts gives off a light hearted , fuzzy feeling too.