I clean after myself, because it is the right thing to do.
Whenever you finish an activity (eating, playing games, doing art, working and so on), it is your responsibilty to clean up after yourself.
If you do not clean up after yourself, you leave more work for someone else (and that is not cool). If you do this often enough, then when people see or think of you, instead of happy thoughts like "Yay, here comes so and so," they will be reminded of negative thoughts like "Oh no, there is so and so, he always leaves a mess behind for the rest of us to deal with." This is what we call leaving a "bad signature" behind you. Don't leave a bad signature behind you.
Further, when we are out in the world, we practice "leaving no trace." This means leaving the spaces we go to (for example a museum, the beach or the forest) just as you found it or cleaner (yes pick up the trash and make it nicer). Think about it, if everyone leaves thier trash everywhere, the world will be an ugly place to look at and live in. The world we live in is beautiful, and we help keep it that way by leaving no trace.
Stay tidy and organized! Once you understand the importance of cleaning up after yourself, you should add the complimentary skill of being "tidy and organized." This means keeping your things in thier right places and finding a home for all the things you own (because things without a home end-up piling and making a mess). When you are tidy and organized, you eliminate wasted time spent looking for your stuff, wasted money replacing lost things, and wasted time doing extra cleaning of your untidy/unorganized mess.
When you are doing a project, and you have a lot of items or tools around you, use the "knolling" technique to help you organize and tidy up the space. Every space you live and work in can benefit from knolling. Here, a famous contemporary artist from New York named Tom Sachs (who learned the technique while working for famous architect Frank Gehry), describes the technique:
Scan your environment for things that are not in use.
Put everything away anything that is not in use. If you aren't sure, leave it out.
Group all like objects.
Align or square (square = place at 90 degree angle) to the surface it rests on.