Maybe i just find it annoying and a tad ridiculous that if i want to play a character with a powerful, kick-ass animal buddy, i have to play a shapeshifting spellcaster. (No, rangers, with only half their level adding to the animal companion, don't count here; it simply isn't a big enough boost to make the companion viable as anything more than a scout.) Sure, PrCs can help a bit here, but every single one, except the Beastmaster, requires you have an animal companion beforehand. But then again, i don't see animal companions as some important, class-defining ability for either the druid or ranger; both have such a host of abilities that animal companions are little more than an add-on.
Sure, if you don't mind watching weeks of hard work go down the drain when you're animal friend bites it in his first combat; normal animals are simply too weak to survive high level combat. I once had a fighter focused on mounted combat who went through over a dozen warhorses by the time he hit 6th level.
This is kind of a sore point for me right now, though; i guess i'm just frustrated. I've spent the last couple weeks trying to make a barbaric, dino-riding halfling build for an Eberron campaign, and it just isn't coming together.