Your average person doesn't get married in their early twenties any more, and so dogs have naturally replaced children for a working adult post-graduate. You finish college, move away to start a new job, and get a dog. Dogs are fun and don't effectively end your current life as you know it for 18+ years like having a kid does, which is why we're seeing this trend. Young people want to enjoy their twenties but still want to have something akin to a child, and dogs have taken that place because they're way lower maintenance and you can actually pretty easily leave them for a weekend, or even a week, to go on vacation, and they can be left at home unsupervised for hours at a time as well. That's obviously ideal for someone young and working.
