Why were dinosaurs protective of their young?

Animals that have adaptations that make them more likely to survive have a higher chance of reproducing and passing those genes on. In the case of dinosaurs that protected their young versus those that didn't, the ones who protected their young would have more offspring that would survive long enough to reproduce, and when they reproduced, they would pass on the behavior of watching the young. In short, protecting their young ensured their genes would be passed on another generation, even though they probably only chose to because they loved their offspring, just like modern mother animals do.