Any union that does not propagate trust as its core value is definitely standing on a shaky foundation. A successful and happy relationship should be predicated on trust, otherwise the union will be punctuated by countless second-guessing, incessant arguing, and an overall unhealthy vibe. Distrust is fueled by insecurity and immaturity, hence one should be hesitant in plunging into a committed relationship if there are any warning signs at all during the dating process.
Have you ever seen one of those Catch-22 couples who keep on pining about a "lack of communication" Its always either one talks too much or one's opinions are constantly ignored. Ironically, these are the types of couples who do a lot of "talking", and yet there is no real communication. Given the intricacies of relationships, this is a skill that is acquired over time through trial and error. In a utopian world, the level of communication should increase the longer the relationship lasts. Unfortunately, this is easier said than done, and there are relationships where couples just "tune out" after a certain number of years. It is said that familiarity breeds contempt, and when contempt is present complacency is not far behind. When a relationship reaches a point where there is no longer any dynamism and most days lean toward the mundane, this is a key indicator that the relationship is headed downhill and is oftentimes a direct result of poor communication between the two parties. The relationship would be dying a slow death, and the tough part is that most couples would never really realize the quagmire that they are in until it is way too late.
Capping off the holy trinity of relationship must-haves is the need for understanding. As much of an acquired skill as an intrinsic personality trait, no relationship would possess any long term viability if there is no semblance of this integral trait. As it is inevitable that there would be the obligatory speed bumps along the way, recognizing the need for understanding shows that a couple is willing to rise above the petty niceties and is more focused on the long term. While not necessarily the easiest thing in the world to do, it is a concept that needs to be exercised with significant regularity in order to avoid failure.
To conclude, it is glaringly apparent that a lack of trust ,communication, and understanding in a relationship are tantamount to a virtual death sentence, as these are two integral components in the dynamic that need to work in full synergy to be effective. Trust issues, while hinging on other factors such as fidelity and experience, can still be negated if a couple could communicate properly. Likewise, well-meaning efforts to communicate will mostly be misconstrued and taken aversely if there is no existing framework of trust, which in turn fuels the level of understanding in the relationship. This is easier said than done though, and would take considerable initiative and force of will to effectively implement. There is also this other intangible that may work in case all else fails though- its that something called "love". It's always a handy thing to have, specially in a relationship