Why should we not re-freeze defrosted meat?

Well the answer is that some bacteria don't harm you only by infecting you: they can actually put things into the food that are calledtoxins,and the toxins are not broken down by heat.

So, bacteria multiplying in the food can lead to the accumulation of toxins in the food, which will then make you ill even though the bacteria themselves that made the toxins may have been destroyed by reheating the food.

And if you repeatedly cool and re-warm food, the food might spend enough time at a sufficient temperature to encourage the bacteria to grow and put toxins into the food whilst not themselves actually really posing much of a threat. That's one way.

Another reasons is that, if you keep on warming up and cooling down food, some bacteria will just end up flourishing and they'll go from being at very low levels in the food, where they're not growing very fast because the temperature is low, to reaching a very high population in the food that might be an infectious dose.

To catch Salmonella, for example, you actually need to eat about a million organisms, 10⁶particles, of Salmonella. That's an infectious dose of Salmonella. Other bacteria infect you at much lower doses. So it really depends on what the pathogen is and the mechanism through which it makes you sick.

The bottom line is, if food spends more time at higher temperatures, there's a higher chance that bacteria will grow and therefore, make you sick. So, the best advice is to either cook it and eat it, or cool it and eat it, but don't keep reheating it, because that could be bad...