There's a lot of reasons, really!
-Animation
It's just fantastic. And ridiculously detailed! Heck, even the movements of the ponies manes are animated correctly! Clearly, heaps of effort gets put into the whole animation process, and that's something to be grateful for in a show.
-Characters
They're down to Earth, believable and lovable ponies. All of us can relate to at least one of them. They all have faults in their personalities, major or minor, obvious or not so obvious. And there's always more to them than meets the eye. Like Pinkie Pie completely losing her mind in Party of One. Or Rarity (who I'm sure not many of us really liked previously) risking her career to please her friends. And the list goes on...
-Songs
The songs in the show are extremely catchy and, occasionally, quite emotional. 'BBBFF' is a good example of an emotional song in the series, and 'This Day Aria' sounds like it came straight out of a Disney movie. Who can't get overjoyed when listening to 'Smile Smile Smile'? And who could go a day without getting 'Winter Wrap Up' stuck in their head? Nopony, that's who.
-Humor
The show is filled with innocent, yet very funny moments which could make you cry with laughter. A lot of this comes from knowing the characters well. For example, after watching a full season of Twilight's generally level head, seeing her completely lose it in Lesson Zero is sure to crack you up. Not to mention Flutterage in 'The Best Night Ever'. But it's filled with other stuff too, like the classic "And then I said, Oatmeal, are you CRAZY?" or Spike demanding another donut. (with extra sprinkles!)
-Morals
Who couldn't love this? At the end of each episode, a life lesson is learned. Most cartoons tend to lack that nowadays, but in MLP, it is summarized clearly, yet not in a way that would be cheesy or annoying. There's a lot that can be learned from the show, and a lot to be taken from it. Freakin. Beautiful.
And that's just to name a few! I didn't even mention the fanbase, great relationship with the creators or the complete adorableness of the characters! But this post is getting way too long.