well first off i'll start off by saying yes i am a brony
now as for the reason, well there's an explanation that's in order, a good chunk of people are saying
"i've seen the show and it's ok but i don't see why grown men are obsessing over it"
and the truth is... they are absolutely right
while the show itself is of good quality, it isn't something that merits going ga ga crazy over, so why are there bronies? the reason varies from person to person, but i believe the main reasoning behind the brony phenomenon is the fandom itself.
i will be the first to admit that if it wasn't for the fandom itself, i might've watched the show once, said "meh" and never gave it a second thought, but the fan produced stuff for the show is most definately one of the most vibrant, entertaining and fun things i have ever seen, i mean sure you've got the REALLY strange stuff (i'm looking at you R34) but that has existed for every fandom so i don't see why ponies should be an exception, but the other stuff such as music, videos or artwork is so numerous and creative, i can't help but like it.
what REALLY makes me proud to be part of the brony community is the charity work. because MLP is focused on kindness, love and tolerance, many a brony charity have sprung up and i have seen these charities raise money for a little girl who is diagnosed with cancer and told she will not make it, but thanks to donations from bronies (as well as Tara Strong) she is doing well so far, as well as a 100% brony started charity that has raised a clinic and shelter in Burundi and an orphanage in Tanzania. how awesome is that!?
human beings are social creatures, we desire to be part of a community, it isn't the cartoon itself that creates the bronies, but the bronies who create bronies, the cartoon is the door, the fandom is the house.