Good recyclers and sincere environmentalists sometimes litter accidentally by letting things outside that can blow away in windy weather or be carried away by creeks or rainwater. (Some thoughtless folks intentionally litter but most of them don't visit recycling websites like this one.) So if you are otherwise an enthusiastic recycler and environmental steward, make sure that you are careful with those sorts of things that could accidentally become litter.
The Trash Free Potomac initiative has a great video suitable for both kids and adults emphasizing the importance of putting things in the right place and preventing both intentional and accidental litter.
Beyond being unsightly, many kinds of litter (but especially plastics) can foul our waterways and end up as ocean pollution that can also be harmful to birds and other marine life. Littered trash is also one of the most common sources of damage to farm equipment.