I think it's a matter of individual taste, some like their tea with a lot of milk, others only have a dash of it, with a stronger brew. Someone I know takes 5 spoons of sugar in his tea! One of my research projects studies the interaction of milk with tea. The variation in taste may be due to the interaction between tea catechins and milk proteins.
Tea polyphenols, which include catechins found in the tea leaf, and their more complex derivatives formed during tea manufacture are responsible for the astringency (strong mouthfeel) of tea. These compounds bind to proteins on the tongue and lining the mouth — that's how we experience the sensation of astringency. In a similar way, the catechins bind to the milk proteins and thereby reduce the astringency of the tea. People who don't like a 'strong' mouthfeel will obviously prefer more milk in their tea. Interested readers should consult Tom Coultate's fine textbook 'Food: the Chemistry of its components'.
The full effect of milk proteins on tea is highly complex. We understand the overall picture but the details are still the subject of active research by tea chemists, including myself. I hope to discuss this further in a future post.