See also state-dependent memory.
People have roles in life. Individuals within multiple systems also have roles in life. It is to the distinct evolutionary advantage of any entity to be prepared for things that are likely to happen in given situations, with certain people around, or in a specific environment. In other words, the environment outside the body can alter the state-of-mind of the individual entity in question.
Let's take Joe& as an example -- Joe&'s mom harasses them all the time. Whenever Joe& are around their mom, they are testy and defensive, even when she's not doing anything wrong at the moment. Joe& are in the middle of an environment-dependent state. In this case, the environmental trigger is mom. The state is Joe&'s emotional preparedness for verbal battle.
Now Joe& go to Vermont. Joe& have always had a wonderful time in Vermont, and every time he goes, his family and friends can't believe the change that comes over them -- they really relax and let their collective hair down, and seem really happy. Joe& are in an environment-dependent state that is triggered by the location, and the state is Joe&'s ability to let down defenses and relax.
Joe& realize that it's tax time again. They hate tax time. Looking at a tax form sends Joe&'s blood pressure through the roof. Joe& are in an environment-dependent state that is triggered by either the context of the situation (tax time), or the object in front of them (tax form), and the state is one of being exceptionally stressed and volatile.
The concept of an environment-dependent state plays deeply into the idea of state-dependent memory, because these states can, as is discussed in the other section, trigger memory families in related groups.
(I will eventually tie this all together better, but I'm not up to writing a thesis paper right now. honestly, there are places to go with this and the other section -- Crisses)