* Thwarted Belongingness: This refers to the feeling of alienation and social disconnection, a sense that one does not belong to or is not an integral part of a valued group (e.g., family, friends, community). Humans have a fundamental need for social connection, and when these ties are cut or perceived as absent, individuals suffer in isolation.
I disagree about the claim that "Humans have a fundamental need for social connection". I believe that humans have a fundamental need for entertainment. A lack of entertainment causes the feeling of boredom. Social connections are just a convenient tool for gaining entertainment, but this tool is not irreplaceable. For example, I could spend days playing some computer games with interesting plot and mechanics without wishing to do anything with people IRL. I only feel the need in social interactions when I'm not busy with some very entertaining process I could deal with alone.
It is important that highly pleasant entertainment commonly requires new impressions. When you complete a game for the first time, you get maximum new information to process and your brain appreciates this a lot. But when you beat the same game for the 1000th time, it won't give you so many fresh emotions and won't be so entertaining as before.
The same applies to interactions with the people around us. When those people give us new impressions, we find the social game with them entertaining. If your interaction with them is limited by similar conversations repeated over and over again, you easily get bored of dealing with them eventually.
Craving for social connections in the absence of them is determined by the feeling of missing opportunities in gaining entertainment from such connections rather than "fundamental need for social connection". Imagine that all people around you just told you that they recently watched a new movie which they call superb and would rate 10/10. You could feel a great disappointment if watching this movie were impossible for you, but this doesn't mean that there is some "fundamental need" in watching good movies. It's just a frustration from a missing opportunity to get a portion of good emotions from some fresh experience.
* Acquired Capability for Suicide: While the desire for suicide (resulting from thwarted belongingness and perceived burdensomeness) is necessary, it's not sufficient for a suicide attempt. Joiner argues that individuals must also overcome their innate instinct for self-preservation and develop a "capability" to inflict lethal self-injury.
I think that the theories about "innate instinct for self-preservation" are too primitive and naive at the same time. I can compare them to the geocentric model of the universe. There were times when people thought that the Sun revolves around the Earth, and this point of view may seem correct at the first glance. If you dive deeply in astronomy, you can notice that this model sucks at explaining the movement of celestial bodies.
Assuming that some biological program is triggered by just the fact of recognized danger of death and then can stop us from committing suicide is somewhat similar to assuming that the Earth is the center of the universe and everything else revolves around it. The real situation is different and more complicated at the same time.
It is an undeniable fact that people have an intrinsic aversion to discomfort (including physical pain that is a particular form of discomfort) which play some role in self-preservation. While discomfort avoidance is helpful for survival of population in general, it's not necessarily helpful in particular situations. It's easy to notice counter-examples showing that discomfort avoidance often leads to self-destructive behavior or even death.
If you're a heavy smoker, your attempts to stop smoking will likely face with severe discomfort which will motivate you to resolve it. The counter-motivation to care about your health will likely lose against the motivation to satisfy the urge in smoking, and then your unhealthy self-destructive behavior continues. So where is "self-preservation" here? Hello, survival instinct, where are you? You realize that your health is in danger, however discomfort avoidance not only doesn't prevent the harmful activity, but directly encourages it.
Severe physical pain and mental distress (boredom, frustration, sadness, etc) can push people towards suicide as the only available method of getting rid of the discomfort associated with such states. Obviously, suicide is quite the opposite to self-preservation, so the role of aversion to discomfort and discomfort avoidance is in fact rather controversial than exclusively life-supporting. Avoiding discomfort is not subordinate to survival (contrary to what self-preservation theories would suggest), and it may actually dominate over survival. When a person is survived due to discomfort avoidance, it's a lucky coincidence. Evolution made such lucky coincidences happening often enough to ensure survival of the population as a whole, but the underlying mechanism remains rather imperfect.
The regret about the loss of opportunities due to death may also contribute to hesitation about committing suicide. But the same regrets might also prevent you from doing non-lethal actions that would result in a similar loss, so it would be premature to blame self-preservation instincts in your attachment to the life. You're attached to entertainment/pleasure in the first place, that automatically makes you value any means which provide you access to these basic values, and life becomes valuable since it belongs to the set of such means.
When attempting suicide, 4 main concerns or problems may arise:
1) short-term discomfort prior to death if it comes within an anticipated time,
2) possible long-term discomfort if suicide attempt fails or results in a much slower and much more unpleasant death than initially anticipated,
3) loss of potential opportunities,
4) despite your decision to CTB, the widespread opinion that death is a bad and tragic thing may influence your intuitive attitude to it.
Some combination of these 4 factors may cause a psychological pressure and counter-motivate you against making suicide done. If this happens, the best thing you can do is to investigate what factors and in what proportion motivated you to stop following your plan; and the worst thing you can do is to blame the fictional monster called "survival instinct", that is not helpful at all. In particular, your level of acceptance of the worst possible scenario may determine how easily you get over the problem #2 on the way of eliminating the fears before CTB.
