Mothers love babies because they get a shitload of attention while pregnant and while they have a baby. babies are also like adorable pets, which everyone loves. Then there's the tax incentive. It takes about 18 years for the novelty and the tax break to wear off, then parents kick their kids out of the house.
Just to make things clear, all I'm doing in the post is saying the argument as offered by Cipher (and many make this kind of argument) doesn't work. However, mother-love could indeed be selfish for other reasons. I myself have been a believer in psychological egoism from my early teens, and I probably still am (though it needs further investigation).
It also has more to do with chemicals than with mothers getting attention or whatever (but what you said it true in some cases, maybe many). But, as is the case with strawberries, chemicals don't mean they're not really sweet. In fact it means the opposite.
But plenty of bad parents out there, true.