Here is why.
First of all, character development. You can CLEARLY see ALL of the characters, including ALL of the side characters, develop. In the ending of this anime, the fates of almost ALL of the characters, even side characters, are told to you. The anime is complete, in every single way.
Every character has a purpose. There is no dillydallying, no characters put in to just hand Ed and Al a piece of information and then disappear for the rest of the series, gone to who knows where. This happens in most animes, but not here. Every character, even the side characters, and EXTREMELY important. If not in the story, then they find some way to enter the hearts of us, whether through connections with other characters who care about them, or whatever else.
There is a perfect balance of comedy and seriousness, sadness and joy, and all other opposites. Character's background stories are executed perfectly, in the right times in the right moments in the right ways.
In the first, let's say... 10 episodes of the anime, (especially first episode), you may find yourself being a LITTLE bit bored, and like "ughh this is like any other anime" but TRUST ME, right after 10-15 episodes it gets to an awesome thing
bye
