Listen before keying up. Also, when I talk to a pilot and as soon as I un-key another pilots calls up with something. Did you think the pilot I was talking to wasn't going to respond? Whether you can hear him or not (your frequency or another), he is responding. Pretend like you are responding...
Haven't seen the complete correct answer posted yet so figured I would help out. Jmcmanna was partially correct, this is a little more detailed. There are two reasons.
1. If you are enroute, the reason is preferential routes/LOAs. The computer system can type a "pref route" on a strip and...