Whether people ask me "why" or laugh and insult me because I like ponies I always give them this:
The animations are really ****ing beautiful, for one thing, and the voice acting is spot-on. The songs are also great, especially if you like when shows or movies randomly break out into song. Another great thing is the comedy, it's not like the comedy of shows like Flapjack that rely on the grime and gruesome imagery to get you to laugh, it's pure comedy and it reminds me so much of oldschool Looney Tunes comedy in a lot of ways, both from physics and smart-alecky comments. The show also gives me a really warm feeling when watching it because of the fantastic bonds you can feel between the friends of the show.
Oh and the facial details are amazing, they are half of what makes the series so likeable because you can just feel the emotions they are trying to convey because of all the facial expressions as well as the body language given.
They certainly don't make it a secret that it's fluffy at times and yet they still keep the humor more generalized rather than just focusing on little girls. It also doesn't do what a lot of shows these days do and focus on social stereotypes and norms, they let everyone in the show be who they are with no regard for social "norms"
Pinkie Pie and Twilight Sparkle going insane? Try to tell me that's something a little girl's show would normally do. How about dropping an anvil on Twilight's head or having Applejack smash into the ground because she's so damn tired she can't jump onto a catapult properly? Even Rarity who lives in the stereotype of the "it-girl" or "material girl" breaks out of the stereotype frequently.
They all have emotions, they all have varying personalities that they constantly break out of for one reason or another and they all have individual lives that aren't cheaply connected.
...More or less, I don't say all of that at one time but whenever I explain it I explain it a bit differently.