I think dog owners should be aware that some people are terrified of dogs (my sister's best friend was badly savaged by a huge German Shepherd dog when young, and literally trembles when she sees any dog coming towards her) Also, many people have allergies and being near a dog (or having one jump up) can set them off sneezing and choking.
Others don't like the slavering that leaves them covered in dog saliva. Or the odour (as Raggytash says).

I personally can cope with all this, and there's no animal I don't like (except spiders!) but not everyone feels the same.

Another point to make is that, as said by Ranger1, such dogs need stimulation and work to do, company and exercise. A couple up our street have just got a young border collie. They're out at work all day, then just take it out for a brief walk in the evening. The poor thing is stuck in a small bungalow, lonely and bored, and now they tell me he's 'getting nervous'. I suspect he's actually slowly going insane. When you see sheepdogs on TV doing such wonderful work, it's tragic that this collie is suffering solitary confinement. If only I wasn't so old and a bit frail, I'd offer to take him out, at least for an amble in the local woods. Why are so-called dog lovers so ignorant?