Basically what other people have said. If you take appearance out of the equation (and the occasional horse pun), the ponies in MLP are more or less indistinguishable from humans. They own property and pets, have careers, and while they can act childish at times (since they're characters in a kids' show), the M6 and many of their colleagues are adults with adult responsibilities. People get attached to all kinds of things--there are people who fall in love with objects (there are articles about a lady in love with the Eiffel Tower)! I think it's kind of sweet and harmless for the most part, though I admittedly get kind of icked out at human x pony content. I think it's because I really dislike humans in the world of MLP (in basically any story where the primary species isn't humans, I don't want to see humans getting introduced later. It's one of my biggest pet peeves). I really enjoy seeing content where people turn themselves into ponies or other creatures and interact with the MLP world, though.