It all boils down to science actually. The way your cuppa tastes, its colour and the physical sensation it leaves its (otherwise known as mouthfeel) depends mainly on two elements of the tea - polyphenols and caffeine.
How these two elements interact with each other depends on the level of acidity that good ol’ H20 contains. Because the acidity level of water is what helps ionize and stabilize the complex that is polyphenols and caffeine. In other words, it brings out the flavour that matters most.
Now as you may remember, water holds certain dissolved gases which it absorbs from the air. One of these gases is carbon dioxide (CO2) which happens to affect the water’s level of acidity and is slowly released during the boiling process. Basically the lesser the level of CO2, the lesser the level of acidity.
What happens when you reboil water is that the level of CO2 gets diminished and so does the acidity which alters the colour and mouthfeel of what should have been an otherwise perfect cup tea.
So the next time you’re about to brew yourself a nice hot cup of tea, remember to always use water that is only freshly boiled once.