DNA doesn't just make you who are you. It can also change its mind about who you are. A particularly unfortunate example of this is social isolation.
The boring informational prelude
Deoxyribonucleic acid - it's not necessarily the 'building blocks' of life. It's more like the instruction manual that tells your body how to build you.
Groups of DNA are called genes, and each gene handles a different function. Many of these functions are unknown. Much is thought to be useless, in fact, and is often referred to as 'junk DNA'. But not all that useless DNA is junk. Some of it is just 'switched off'. For example, we all have a gene for a tail in our DNA, but only a tiny proportion of us will end up with that gene turned on.
Our bodies literally rebuild us in response to the world.
So some of our genes are 'off' and the ones that are 'on' get used to build us. We used to think that those genes that were off, stayed off. But a recent area of science has been created around the discovery that some genes seem to turn on and off due to changes in the environment - it's called epigenetics and it has powerful implications for our behaviour.
You see, one of these environmental changes that can mess with our DNA is social rejection. It has been found that isolating experiences can actually switch on genes that change any new cells created after that experience.
This isn't an immediate change though. Much epigenetic research (usually on the effects of exercise) finds that it takes about six months for the new gene's functionality to have swept through the body. So, social isolation, if prolonged enough, will literally make you a new person. But, what kind of person?
Social exclusion makes us literally sick
Studies indicate that following an experience of social rejection, some of the genes involved in resisting viruses switch off (the scientific terminology is down-regulation) as soon as 40 minutes following the incident. Alongside that, some of the genes in charge of our inflammatory response (part of the wound healing process) 'turn on' (up-regulate).
So we're more likely to be unable to resist things like colds and flus. And when our inflammatory system gets too excited, it can get confused and start attacking your own cells (thinking they're invaders).
Obviously this isn't going to happen the next time someone refuses to go with you to the school dance, but over time with a cumulative effect this becomes a problem (and more people than you realise are at risk of these cumulative effects).
I think it was Lance Armstrong who said "boos are much louder than cheers" (I bet he's been a bit overwhelmed lately) and it's certainly true that negativity tends to hold our attention better in general. But it's thought that this particular sort of response at such a basic biological level is a leftover evolutionary function. Preparations for what you might face if you were kicked out of the herd. Less resistance to viruses because you're no longer around other people - the main place viruses do business. Your body doesn't need to waste energy protecting you from your sickly comrades any more. Heightened inflammatory response because you're more likely to get eaten. Better wound healing response means you're more likely to survive an attack.
The crux - why should you care
There's a deeper layer here that I want to touch on. It's the idea that the need for social support is so fundamental to us, that it's genetic. That our social status is such a critical factor to our well-being, it can alter us at the most basic level in response to perceived threats. Considering how young the study of epigenetics is, one wonders just how deep this particular rabbit hole goes.
I gave him my heart, and he took and pinched it to death; and flung it back to me. People feel with their hearts, Ellen, and since he has destroyed mine, I have not power to feel for him.” ― Isabella, regarding Heathcliff, Wuthering Heights