the characters have enought tiem to be developed and you know how they feel and act,know their personalities,advantages and disvantagens and you care for them for example pink pie is not only full of happiness you pink turning sad and how she reacts and therefore care for hers because she look like a living being with emotions and one of the reasons some characters from EG don´t have a defined personality(flash) its because they didn´t got enough time on the scream.
MLP FIM has good life lessons that are given in a fun form that can get your attention and the lessons are really valuable (in the majority of times i think) i will give you an example the episode For Whom the Sweetie Belle Toils shows us that not everytime we are going to get the attention we want but its not righ to blame the one with the attention and sometimes we need to accept that
3-situations(warning of G3 references)
the situations in the show are ones we can relate with and undestand for example in the episode return of harmony rainbow dash get out of the labirint because she though her home was going to be destroyd and we can understand that because who wouldn´t try to save your home and for an example i wil tell the story of the episode run away rainbow dash(obs:this is not an FIM episode this is a G3 click this link for more information ) where dash run away because she lost his colors...and lets be honest that is ridicoulous
okay that was it my 3 reasons there is of course more reasons but there this was only i could think of so if you have other opnion post on the comments