In Linguistics we have been studying how language changes. Something interesting about English, Modern English at least, is the fact that we only have one form of the second person pronoun, the word you. Most other languages have a separate term for plural, and possibly even a different word when showing respect.
English used to have these forms: thee, thou, and thy were singular forms, and ye, you and your were the plural forms. The plural forms were used to show respect. When addressing a superior one would use you and that person would respond using thou. The use of thou also showed familiarity; one would use thou when speaking to family members, but use you when addressing strangers.
Because of major social changes (urbanisation, the rise of the Middle Class) people didn't know who was who anymore. They couldn't tell who to be polite to, so they were just polite to everyone, using the plural form in all contexts. It then became standard to refer to all strangers as you, whatever the hierarchy. Eventually, we lost the familiar thou altogether.
Thou does still exist, in religious contexts. The reason for this is interesting. We think that thee and thou show respect, when in fact they were originally used to show familiarity; these words were used when referring to God because God is our father. Now we hear thou used in some older versions of the Bible and we think it is because when speaking to God we ought to show respect. It just shows how we keeping distancing ourselves from God. It's the same with the word abba. We know that abba means "father", but (I don't know about you) when I say abba I (used to at least) put that same sort of respectful distance into the word. Remember that song that goes: "You are awesome in this place, Mighty God. You are awesome in this place, Abba Father..."? In fact, abba is the Hebrew equivalent of daddy.
We think we ought to be proper and perfect, call him sir and never step a foot out of line, when all God really wants is to be our dad. He wants us to call him daddy, to know him well enough to use thou. To run into his arms full of dirt and grass-stains and tear-filled eyes and tell him we fell down again, even though he has told us a million times not to run so fast. To bring him all our failures and mess-ups and let him kiss them better. To not be afraid to come to him before we are perfect, and trust him to love us completely despite the dirt and the brokenness.