Why do we ALWAYS have neighbor issues?

So, our neighbor across the street apparently thinks it's ok to dump his yardwaste onto our property. Why is it that people think it's ok to do these things? Who dumps their CRAP onto other peoples property? I just don't get it. And then, why is it always us? This was the stupidness that led to the whole mess at our old house!! UGH! What do I do now? I really want to pick it all up and dump it back onto their property but I know that would be childish (would feel good though).