When I read the paragraph below:

In CBT difficult feelings such as sadness (and anxiety) are seen not only as normal, but also as healthy reactions to the environment and life in general. However, when someone experiences such feelings in their most extreme, they can develop avoidant behaviour (for example, stop engaging in daily activities), which will inevitably distance them from family and friends; in turn, causing feeling of detachment that in most cases leads to suicidal thoughts.

I was reminded of someone from my hometown. About five years ago when I last visited home, I noticed that he kept a lot to himself. He lived in a big family house with hus siblings but he neither spoke nor associated with them most of the time. When I saw him this time, he had stopped opening his mouth entirely. When asked, the reason he offered for keeping his mouth completely shut was that people would poison him if he dared open his mouth long enough to speak. His isolation from the community started when his mother died of unknown causes. He chose to believe that it was his step mother and her children that were responsible for his mother's death. I'm not sure but I think the negative thoughts were in a spiral like you said:

Negative thoughts > triggering low mood > prompting more negative thoughts > lowering the mood further, and so back to the beginning.

I sometimes feel depressed and in those moments I have no desire to speak to anyone including those who care about me. I avoid them. What I found out, however, is that negative feelings beget negative feelings and so on. So for example, on my birthday, I felt sad because of the the things I had failed to achieve, so what I did was to recall all the things I did achieve and all the blessings I've had including the important people in my life. This helped me dissipate the negative feelings and a miracle happened - the positive feelings also came in a flood. So I believe that,

it is not so much the situation that prompts emotional reactions; but rather the meaning that the person concerned associate to it.

Two people can gave the same experience and while one would allow the experience to weigh him or her down, the other person may choose to see it as a challenge that can be overcome, an opportunity for growth. I am that second person.

You make psychology so interesting. Thank you for this beautifully written post. Pay no attention to my typos: I just couldn't wait to send you my thoughts.

Awesome!