Why do people say drinking milk from other mammals are bad for us?

I don't see a problem with it,did I suddenly mutate to a cow or have terrible illness? No, humans have been drinking milks for centuries because its very accessible and cheaper ways to get a drink from other mammals and they've turned out fine. Yes they're will be those smart-*** people that will try to prove me wrong that drinking milk is bad no I am not agasint them at all if you they don't want to drink milk so beat it go drink soy or rice milk then all I'm asking is,why do people say drinking milk from other mammals are bad for us?