I remember reading somewhere that it is due to human babies having large heads, plus walking upright necessitates an awkward birth canal. These factors combine to make human birth unpleasant all around. I don't know if that's correct!
I don't see why one would think it not unpleasant for animals as well? Most animals don't show pain like humans do.
My parrot once got an egg stuck and DAM did that hurt her! She plucked her swollen ass bold and tried to run away, we had to put her in an incubator and rub her ass with Vaseline, when she finally expelled that egg she was soooo happy, making happy noises and picking my nose.
You see most animals express their pain differently then humans do.
Dogs make crying noises and yelp when they are in pain. Dogs may come out at birth making crying noises but still the human seems to have a harder time. This can be just we are more aware of human expression and humans are more expressive overall. I was just thinking about the facial expression of just born humans and you wouldn't call it happy.