English and (I believe) Brazilian Portuguese have to varying degrees lost T-V distinction via adoption of the formal second-person pronoun for both formal and informal situations. English completely lost "thou" and "thee", and Brazilian Portuguese largely lost "tu". I ask because modern languages seem to be gradually expanding use of the informal second-person pronoun to cover more situations traditionally using the formal, losing T-V distinction in the opposite way.
It's easy to imagine a cultural decrease in formality leading to a protracted loss of the formal pronoun, but it's fairly difficult to imagine the forces that would lead to a loss of the informal. Was there a period of increasing formality that saw loss of the informal pronoun in English and Brazilian Portuguese?