Considerations, my friend. If I may borrow the term "game" from Scientology without borrowing their definition or any of their ideology, then all troubles ever had by anyone is the result of a "failed game", or a lack of control.
Most people engage in innumerable games throughout their lives. Some intelligent, some unintelligent. Almost everyone at some point or another involves themselves in the latter, an unintelligent game, or a game which requires too much skill, knowledge, or intelligence to succeed. Everyone who has lived long enough to make decisions, that is.
Any game we can play which is too easy is usually ditched and replaced with a game of the appropriate level of difficulty; any game we play which is too challenging is often ditched only *after* it has caused us much grief - heck, if ever. Some of us play games which are too difficult our entire lives.
To have a reasonable game, we would ideally be equal parts cause and effect in that game. Balance between us being agents of cause and effect would allow us to play without excessive control, in which game we would be too much cause, which would result in predictability and boredom; or insufficient control, in which game we would be too much the effect to other causes and which would result in loss and subsequent negative emotions. Anytime a person has a grievance of any sort, s/he can always trace it back to an unsuccessful try at a game, even if s/he has forgotten the moment during which s/he entered the game and/or that s/he is playing.
If we overstate our ability or carelessly enter into a game without thoughtful consideration of whether we want to participate or assessment of our ability to succeed in it, we will likely lose and experience a negative emotional reaction.
Our minds are in many ways like physical reality, miniature replicas, or microcosms, if you'd like. "Nature abhors a vacuum" should be revised as "Nature abhors inconsistency" - which is a statement that encapsulates the former by its generality - because that is exactly what is happening when we lose a game: inconsistency. When this happens, to clarify, our mental model is in some way out of accord with objective reality. There is an inconsistency between the subjective and objective. We aren't playing by the rules of objective reality and therefore are unhappy. While we aren't expropriated of our free will by any agent of any game within the confines of a particular game, we are certainly unhappy if we defy the reality of things!
Now how to fix this? Because the outcome of any given game utterly depends on two sets of conditions (those of objective reality and those of a person's subjective reality), there are two variables we can change to affect the outcome, and thereby create happiness: we can change the subjective which is always amenable (such as the decision to participate in any given game to begin with), or we can influence the objective within the parameters of what is unfixed and hasn't already been considered on a meta-level, which is/are the rule(s) of the game and/or any action(s) by other players.
Considerations, the whole of which makes up a player's subjective reality, can be as mundane and innocuous as the unnoticed decision that you'll make another person fall hopelessly in love with you and you'll ride away into the sunset. That you'll ride to work in your automobile without troubles. It might be an inherited morality. A set of norms. Or your own personal identity; considerations about who and what you are as a person. These often have the apparency of realness on par with that of objective reality. But all considerations can be identified and modified.
Surely if a breakup occurs in the romantic relationship described above, either by decision of one or more of the players or by a meta-level decision (e.g., non-suicidal death of the other player), that player might experience grief, rage, or bereavement - all appropriate reactions to such a significant game, which we decided to play, to live interminably and happily ever after. Whose fault is it that one or both of the players was/were indoctrinated with such romantic notions as everlasting love in childhood (courtesy of Disney)? That relationships always work? That people are always ethical? Or kind? Or the acceptance of the consideration that death is always (or ever) a lugubrious occasion, a time for mourning?
It's worth noting that sometimes we even participate in two or more games which are mutually exclusive - i.e., a win in one (or more) necessarily equates to a loss in one or more other games. Playing two incompatible games is a decidedly unintelligent thing to do, which always results in poor consequences. Nature abhors inconsistency - whether it's because of your own considerations clashing against each other or one or more of your considerations clashing up against a meta-level consideration (a rule), such as trying to control that which is controlled by another player and therefore out of personal control.
Of course, we can improve our influence over the objective, but that means we must constantly learn or uncover the rules and actions of other players (other humans, reality, etc.)
(To be clear, I don't always mean "external" when I say "objective", which is an entirely different distinction, but objective as in the reality of things. A person who, say, hates brussels sprouts does consider that brussels sprouts are terrible, which is subjective, a consideration, a decision. But the fact that s/he hates them is objective because it refers on a meta-level to a consideration that brussels sprouts are bad.)
One should always have a healthy awareness of what is objective and what is subjective, and exclude assumptions. Because any consideration or action we take belongs to a subset of possibilities determined on a meta-level, sometimes considerations about reality become personal reality. It either collides with meta-level constraints or limits them.
Now it should be apparent what the game plan is for remedying any situation of unhappiness: identify the game(s) being played and all associated considerations and make the appropriate adjustments. A person is only as happy is s/he is free to shift her or his subjective reality. He or she only needs to be able to change their mind.
"I think I can tentatively explain the reasoning behind this and the dynamics behind why stress builds in the first place more in-depth if needed"
I realize that the initial statement that I'm explicating (directly above) describes a two-fold partition between the causes of stress (unhappiness) and the reasoning behind "this" (quoted from above), which actually refers to the nature of truth, verisimilitude, and the "soul". To avoid making this dreadfully long post any longer, I'll withhold speculations about the second group of subjects until some unknown time in the future.
--Brandon