Your carry-on bags should be of a small enough size to fit under the seat in front of you if there isn’t room in the overhead storage compartments. It’s further advised you should use the compartment closest to your seat. Some people are overhead bin hogs. They keep getting up and down during the flight to retrieve or put things back in their bag overhead. This can be disruptive to both the passengers and the crew who are serving food and drinks and attending to the other passengers. Then there’s the guy who puts his stuff in the bin closest to the front exit, so he doesn’t have to hold it waiting to get off the flight. This throws everyone else off, both with storage and exiting time. The courteous thing is to respect the finite amount of space in the overhead bins, and your small piece of it.