A race track is a large place. 4+ kilometers of asphalt, 15 meters wide. A MotoGP bike is a small thing, under 2 meters long from nose to tip, and 60 centimeters wide. The bikes should get lost in the vast expanse of asphalt on track. Yet somehow, these tiny vehicles always seem to run across each other on track.
The riders are to blame, of course. There are advantages to be gained from following other riders around. In Moto3, a slipstream is vital to gaining extra speed. In MotoGP, using a rider ahead as a target allows you to judge your braking points better and gives that extra bit of motivation which is worth a tenth or two. And a tenth or two can mean starting a row ahead of where you would otherwise.
When bikes meet on the track, it always sparks resentment. The rider in front is annoyed at being followed, and will slow down to try to force the other rider in front. The rider behind gets annoyed by the antics of the person they are trying to follow. In the best case, it is all soon forgotten. In the worst case, well, it involves Marc Márquez and Valentino Rossi and a small war breaks out in the Italian and Spanish press, and a much bigger war breaks out among the fans.