There's a lot to be said for the unconditional love of a pet. They love us and are there for us no matter what, which is not always the case with other humans.
However, I think that maybe people who turn solely to the companionship of an animal over having human relationships have maybe had bad experiences with humans. Pets won't cheat on you, betray your trust, leave you, etc.
But I do think that, in general, pets can help us enhance our relationships with humans. Many of my current friends I actually met in one way or another because of my dog! I always met new people while out walking my dog and am much more social now than I was before I got him. Pets, for the most part, become part of our families. Loving them doesn't mean we stop loving the humans in our lives.