Why do many people have the 'urge' to crossbred their pets?

I saw that many people own crossbred pets(especially dogs),lets say,lab-rott,pom-pin,and many others,and they often crossbred their own dogs for personal satisfication. Many people seem like to see their different kind of dogs to reproduce and produce offspring(non-purebred). What are the reasons behind this behavior? Is it because of human’s silliness? Curiosity? Or simply because of carelessness?
Is this the fact that human/most human like to play as Darwin? Why less people value purebred pets/dogs?